Limiting sig pic sizes?

A few posters have been complaining recently about how they can't add sig pics. I was worried about the sizes of the things getting out of hand. Ideally I'd limit the size to.. whatever the full width is x 100 or somewhere around there. For now, I can smack users who attach overly large images, but how would I go about putting size restrictions (both pixels and file size) on sig pics?

Thanks.