Based on my knowledge of Django's codebase structure, I'll create the patch. The issue is to add a check that ensures `max_length` is large enough to fit the longest value in `choices`. This check should be added to the Field class in `django/db/models/fields/__init__.py`.

--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -308,6 +308,25 @@ class Field(RegisterLookupMixin):
                     )
         return []
 
+    def _check_max_length_fits_choices(self):
+        """
+        Check that max_length is large enough to fit the longest choice value.
+        """
+        if self.max_length is not None and self.choices is not None:
+            choice_max_length = 0
+            for choice in self.flatchoices:
+                value = choice[0]
+                if isinstance(value, str):
+                    choice_max_length = max(choice_max_length, len(value))
+            if choice_max_length > self.max_length:
+                return [
+                    checks.Error(
+                        "'max_length' is too small to fit the longest value "
+                        "in 'choices' (%d characters)." % choice_max_length,
+                        obj=self,
+                        id='fields.E009',
+                    ),
+                ]
+        return []
+
     def _check_choices(self):
         if not self.choices:
             return []