Better Image Upload for Advertisement Products

When trying to sell ad space, there does not appear to be any to specify the exact size of an image.  I see you can set a maximum size, but that would allow people to upload a 20x20 box where a leaderboard is designed to fit on the site. This seems like a strange approach and easily fixable by also allowing setting a minimum size.  Of course and exact size or ratio setting would also be great.

Unless I am missing something here.