I was just about to post the same thing. LOL
I would like different sizes also. I added 3 categories...blinkies, avatars, & siggys to our shop. I don't like only having 1 size because the blinkies on average are 150 x 20 pixels...avatars 100 x 100 & the siggy size we use is 300 x 300.
With only having the option to set 1 size, 1 or more of those items isn't going to look good because it'll be either too big or too small in the preview.
Is there a way to have different sizes for different categories?