diff --git a/src/bootstrap5/renderers.py b/src/bootstrap5/renderers.py
index 7ec074c2..ee9c99a9 100644
--- a/src/bootstrap5/renderers.py
+++ b/src/bootstrap5/renderers.py
@@ -285,6 +285,7 @@ def add_class_attrs(self, widget=None):
             classes = add_css_class(classes, "form-check-input", prepend=True)
         elif isinstance(widget, Select):
             classes = add_css_class(classes, "form-select", prepend=True)
+            classes = add_css_class(classes, self.get_size_class("form-select"))
 
         if self.field.errors:
             if self.error_css_class: