I've only seen one size of Build a Bear clothes. Stewie has about 10 shirts from there.
Here is one photo wearing his "Mother's Day" shirt:
http://img.photobucket.com/albums/v5...Picture096.jpg http://img.photobucket.com/albums/v5...e/52edaa1e.jpg
Here's my BF's niece with her two Build a Bears that we made & their shirts:
http://img.photobucket.com/albums/v5...gsandbears.jpg