Yes, it's because of the size, avatars can be max 64x64. But you don't need to make a separate uppload for it, just edit the one you already submitted to have this cropped version as preview icon and set the full artwork as the big file.
I tried to use the other one as an Avatar but didn't let I tought it was because of the size. Any idea why?
Rather than making a separate upload, you can edit the original uload to have this set this as the "preview icon", and the full 100x100 version as the big file.
Ok, perfect