Upload & Sell: On
As a fine art photographer and printmaker, I allow the image to tell me the size it wants to be, and the combination of the original format used along with the appropriate cropping for composition will determine the final aspect ratio. Remember, cropping is your most powerful compositional tool. It's staggering how many people ignore it's benefits. And what about panoramas, either horizontal or vertical? Trying to fit your work into a predetermined restriction such as 2:3 or 4:5 is strangling.
I agree with all of this 100%, but as you said, this is from the perspective of the photographer/printmaker.
I think it's absurd to say there is a 'popular' print size or a 'favorite' aspect ratio.
From the customer standpoint, I am sure certain trends could be identified if we had access to enough data on sales. Popular or favorite may not be the right word, but I would not be surprised if the data showed an apparent preference for one general aspect ratio over another.