Allowing Only Thumbnail Size Images In Posts

Hi,

Is there a plugin, or code to help me create a plugin, that when an author or contributer on my site is creating a post, uploading an image and choosing the option to add that image to the post . . . would allow only the thumbnail sized option to show (be chosen) or somehow block the user from choosing any other size than the thumbnail size to place in the post? I definately want to stay away from editing core files for obvious reasons. I update my wordpress site as new updates come out so that would be really difficult. Also, even though I can edit, php is not my strong suit.