diff --git a/resources/views/admin/user/edit-shirt.twig b/resources/views/admin/user/edit-shirt.twig
index 37705b01..ab3da7a0 100644
--- a/resources/views/admin/user/edit-shirt.twig
+++ b/resources/views/admin/user/edit-shirt.twig
@@ -20,6 +20,9 @@
                     <div class="col-md-6">
                         {{ f.select('shirt_size', __('user.shirt_size'), config('tshirt_sizes'), {
                             'selected': userdata.personalData.shirt_size,
+                            'required': true,
+                            'required_icon': true,
+                            'default_option': __('Please select...'),
                         }) }}
                     </div>
                 {% endif %}
diff --git a/resources/views/pages/settings/profile.twig b/resources/views/pages/settings/profile.twig
index 83f7b470..b6db1a70 100644
--- a/resources/views/pages/settings/profile.twig
+++ b/resources/views/pages/settings/profile.twig
@@ -143,6 +143,7 @@
                         'selected': user.personalData.shirt_size,
                         'required': true,
                         'required_icon': true,
+                        'default_option': __('Please select...'),
                     }) }}
                 </div>
             {% endif %}
diff --git a/src/Controllers/Admin/UserShirtController.php b/src/Controllers/Admin/UserShirtController.php
index 807a607f..d213b0bc 100644
--- a/src/Controllers/Admin/UserShirtController.php
+++ b/src/Controllers/Admin/UserShirtController.php
@@ -58,13 +58,13 @@ class UserShirtController extends BaseController
         $user = $this->user->findOrFail($userId);
 
         $data = $this->validate($request, [
-            'shirt_size' => $shirtEnabled ? 'required' : 'optional',
+            'shirt_size' => ($shirtEnabled ? 'required' : 'optional') . '|shirt_size',
             'arrived'    => 'optional|checked',
             'active'     => 'optional|checked',
             'got_shirt'  => 'optional|checked',
         ]);
 
-        if ($shirtEnabled && isset($this->config->get('tshirt_sizes')[$data['shirt_size']])) {
+        if ($shirtEnabled) {
             $user->personalData->shirt_size = $data['shirt_size'];
             $user->personalData->save();
         }
diff --git a/src/Controllers/SettingsController.php b/src/Controllers/SettingsController.php
index 6e1ec4e7..3c9dd5bb 100644
--- a/src/Controllers/SettingsController.php
+++ b/src/Controllers/SettingsController.php
@@ -296,7 +296,7 @@ class SettingsController extends BaseController
             $rules['planned_departure_date'] = 'optional|date:Y-m-d';
         }
         if ($goodie_tshirt) {
-            $rules['shirt_size'] = 'required';
+            $rules['shirt_size'] = 'required|shirt_size';
         }
         return $rules;
     }
diff --git a/src/Http/Validation/Rules/ShirtSize.php b/src/Http/Validation/Rules/ShirtSize.php
new file mode 100644
index 00000000..325045b5
--- /dev/null
+++ b/src/Http/Validation/Rules/ShirtSize.php
@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Engelsystem\Http\Validation\Rules;
+
+use Respect\Validation\Rules\In;
+
+/**
+ * Validates whether the value is contained in the keys of the config item "tshirt_sizes".
+ */
+class ShirtSize extends In
+{
+    public function __construct()
+    {
+        parent::__construct(array_keys(config('tshirt_sizes')));
+    }
+}
diff --git a/tests/Unit/Controllers/Admin/UserShirtControllerTest.php b/tests/Unit/Controllers/Admin/UserShirtControllerTest.php
index 38a217ae..0c6a9fb2 100644
--- a/tests/Unit/Controllers/Admin/UserShirtControllerTest.php
+++ b/tests/Unit/Controllers/Admin/UserShirtControllerTest.php
@@ -7,6 +7,7 @@ namespace Engelsystem\Test\Unit\Controllers\Admin;
 use Engelsystem\Config\GoodieType;
 use Engelsystem\Controllers\Admin\UserShirtController;
 use Engelsystem\Helpers\Authenticator;
+use Engelsystem\Http\Exceptions\ValidationException;
 use Engelsystem\Http\Redirector;
 use Engelsystem\Http\Validation\Validator;
 use Engelsystem\Models\User\PersonalData;
@@ -60,6 +61,7 @@ class UserShirtControllerTest extends ControllerTest
     }
 
     /**
+     * @todo Factor out separate tests. Isolated User, Config and permissions per test.
      * @covers \Engelsystem\Controllers\Admin\UserShirtController::saveShirt
      */
     public function testSaveShirt(): void
@@ -81,11 +83,11 @@ class UserShirtControllerTest extends ControllerTest
             ->create();
 
         $auth
-            ->expects($this->exactly(6))
+            ->expects($this->exactly(5))
             ->method('can')
             ->with('admin_arrive')
-            ->willReturnOnConsecutiveCalls(true, true, true, false, false, true);
-        $this->setExpects($redirector, 'back', null, $this->response, $this->exactly(6));
+            ->willReturnOnConsecutiveCalls(true, true, false, false, true);
+        $this->setExpects($redirector, 'back', null, $this->response, $this->exactly(5));
 
         $controller = new UserShirtController(
             $auth,
@@ -131,7 +133,12 @@ class UserShirtControllerTest extends ControllerTest
                 'shirt_size' => 'L',
             ]);
 
-        $controller->saveShirt($request);
+        try {
+            $controller->saveShirt($request);
+            self::fail('Expected exception was not raised');
+        } catch (ValidationException $e) {
+            // ignore
+        }
         $user = User::find(1);
         $this->assertEquals('S', $user->personalData->shirt_size);
 
@@ -142,6 +149,8 @@ class UserShirtControllerTest extends ControllerTest
                 'arrived'    => '1',
             ]);
 
+        $user->state->arrived = false;
+        $user->state->save();
         $this->assertFalse($user->state->arrived);
         $controller->saveShirt($request);
         $user = User::find(1);
diff --git a/tests/Unit/Http/Validation/Rules/ShirtSizeTest.php b/tests/Unit/Http/Validation/Rules/ShirtSizeTest.php
new file mode 100644
index 00000000..9186d31d
--- /dev/null
+++ b/tests/Unit/Http/Validation/Rules/ShirtSizeTest.php
@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Engelsystem\Test\Unit\Http\Validation\Rules;
+
+use Engelsystem\Http\Validation\Rules\ShirtSize;
+use Engelsystem\Test\Unit\ServiceProviderTest;
+
+class ShirtSizeTest extends ServiceProviderTest
+{
+    private ShirtSize $subject;
+
+    public function setUp(): void
+    {
+        $app = $this->createAndSetUpAppWithConfig([]);
+        $app->get('config')->set('tshirt_sizes', [
+            'S' => 'Small Straight-Cut',
+            'M' => 'Medium Straight-Cut',
+        ]);
+        $this->subject = new ShirtSize();
+    }
+
+    /**
+     * @return array<string, array{string, bool}>
+     */
+    public function provideTestValidateData(): array
+    {
+        $data =  [
+            'empty string' => ['', false],
+            'null' => [null, false],
+            '0' => [0, false],
+            '"S" (known value)' => ['S', true],
+            '"M" (known value)' => ['M', true],
+            '"L" (unknown value)' => ['L', false],
+        ];
+
+        return $data;
+    }
+
+    /**
+     * @covers \Engelsystem\Http\Validation\Rules\ShirtSize::__construct
+     * @dataProvider provideTestValidateData
+     */
+    public function testValidate(mixed $value, bool $expectedValid): void
+    {
+        self::assertSame($expectedValid, $this->subject->validate($value));
+    }
+}
diff --git a/tests/Unit/Http/Validation/Rules/UsernameTest.php b/tests/Unit/Http/Validation/Rules/UsernameTest.php
index 47b9f84b..705d9745 100644
--- a/tests/Unit/Http/Validation/Rules/UsernameTest.php
+++ b/tests/Unit/Http/Validation/Rules/UsernameTest.php
@@ -4,7 +4,6 @@ declare(strict_types=1);
 
 namespace Engelsystem\Test\Unit\Http\Validation\Rules;
 
-use Engelsystem\Application;
 use Engelsystem\Config\Config;
 use Engelsystem\Http\Validation\Rules\Username;
 use Engelsystem\Test\Unit\ServiceProviderTest;
@@ -20,11 +19,8 @@ class UsernameTest extends ServiceProviderTest
     {
         $this->subject = new Username();
 
-        $app = $this->getApp([]);
-        $this->config = new Config([]);
-        $app->instance('config', $this->config);
-        $app->get('config');
-        Application::setInstance($app);
+        $app = $this->createAndSetUpAppWithConfig([]);
+        $this->config = $app->get('config');
 
         // load "username_regex" from the default config
         $defaultConfig = include __DIR__ . '/../../../../../config/config.default.php';
diff --git a/tests/Unit/ServiceProviderTest.php b/tests/Unit/ServiceProviderTest.php
index cd9c5afe..8af9ab79 100644
--- a/tests/Unit/ServiceProviderTest.php
+++ b/tests/Unit/ServiceProviderTest.php
@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace Engelsystem\Test\Unit;
 
 use Engelsystem\Application;
+use Engelsystem\Config\Config;
 use PHPUnit\Framework\MockObject\MockObject;
 
 abstract class ServiceProviderTest extends TestCase
@@ -15,4 +16,18 @@ abstract class ServiceProviderTest extends TestCase
             ->onlyMethods($methods)
             ->getMock();
     }
+
+    /**
+     * Creates an Application instance with a Config set as 'config'.
+     * Also sets up the instance as global Application instance.
+     *
+     * @param string[] $methods Names of the methods to mock
+     */
+    protected function createAndSetUpAppWithConfig(array $methods = ['make', 'instance']): Application|MockObject
+    {
+        $app = $this->getApp($methods);
+        $app->instance('config', new Config([]));
+        Application::setInstance($app);
+        return $app;
+    }
 }