Eilidh, actually I have separate listings for each size in my shop, and I don't think it looks too saturated.... :)
If you have enough designs, you can rearrange your shop so that duplicate designs are spread out across your pages.
Also you can vary the first photos in each listing to avoid "sameness"....
For example, with this listing:
http://www.etsy.com/listing/90479243/ballerina-childrensbaby-art-print• Small prints could use just the image (unframed) for the 1st photo.
• Medium prints could use a cropped close up of the monster, for the 1st photo.
• Large prints could use the framed image on the wall, for the 1st photo.
Also, my shop categories are divided into small, medium, and large prints, to (hopefully) make particular sizes easier to find.
In my listing descriptions, I use both inches and metrics to measure my prints, although I print them in US standard sizes 5"x7", 8"x10", 11"x14" etc...
A lot of sellers list 8"x8" and 5"x5" prints, and are very successful with selling them.... but I've personally had difficulty finding ready-made square frames where I live.
I recently went to 3 art supply stores, and only found 2 brands of square frames among the gazillions of rectangular frames. :/
4x6, 5x7, 8x10, 10x13, 11x14, 16x20 seems to be the standard in US frames... unless you buy frames custom made.
Outside the US, however, square frames/prints seem to be really popular. LOL :P