I ordered a large one and the standard dimensions are correct 3.25 across and 4.50 tall. The inside is white and the printing looks great, good color reproduction and details. There is nothing printed on the bottom, no care instructions nor artist name. Overall, I'm very pleased with the quality out of the box.
Somehow I screwed up and didn't size my image correctly (I think I had two windows open), so I ended up with the blank background edges. I wish we could get the 'look at all sides' display in the pricing window, rather than having to jump out to another tab to see it after settings are saved.