I don't mind just selling one size. If I can keep most of the money I make selling images, I can lower my prices a bit and it's quite easy for buyers to resize images if they want to using free software, like irfanview. Graphic Leftovers only sell one size and they seem to be doing better than other sites that started around the time they did.
There are a few problems selling direct using paypal, the legal stuff is a big one but I'm sure someone will set up a way to do this. That might be why some sites are cashing in now, I really don't see us taking such low commissions forever.