Upload & Sell: On
Yup, almost no one into photography still uses small screens. That it so 1990's...
For the vast majority of us 640 is a freaking postage stamp and 800 is still really really small - Too small to see any detail no matter what screen rez one is using.
1600 is the limit for FM hosted images. That doesn't mean you have to format all your images to that size. The lack of any limits for off-site images and the sizable hosted limits just means that YOU get to decide instead of being forced like a grade-schooler. Common sense still applies. As mentioned, the results from a poll taken about a year or two ago showed that out of the 1,000 or so people who participated the most desirable viewing size was 1200 in landscape orientation. There were almost no votes for anything under 1,000 so from that you can deduce that 95% or more like images somewhere between 1,000 and 1,200 pixels across in landscape.
Now if you go to a gamer site or a site like XDA then their screen grab preferences will be quite different I'm sure. But for photographers... The masses have spoken. Hehe... BTW, most browsers will allow you to size down the images with something like (Ctrl - ) and back up with (Ctrl + ) when done. So there's a solution for those few folks out there still using tiny screens.
I dig "people" photos with a little detail in them BTW:
1000 x 562 + whatever the frame adds
1100 x 825 + whatever the frame adds
1000 x 750 + whatever the frame adds
1200 x 765 + whatever the frame adds