Hi,
The sizes depend on your pixels. Unlike other sites we never crop or skew images to fit standard sizes, instead allowing the artist free scope to just upload what they have
The only limits we have are that one of your dimensions is going to be forced to fit the following list:
8"
10"
12"
14"
16"
20"
24"
32"
36"
48"
The other dimension will be scaled proportionally to maintain the aspect ratio of your image.
We only need 100 pixels/inch (100ppi) in order to have a nice image for printing. That makes the math easy as well. Your image menu can be viewed as a pixels/inch ratio, and you can see how many inches wide by tall your image is. ie 10000 would be 100" 1000 would be 10"