a standard size would be 1600 x 1200 or 3000 x 2250 at the largest uploadable size. a widescreen image would be 3000 x 1875 at it's largest size or any reduction of that proportion.
for a long time, 1600x1200 was the large\standard that most artists posted ... even though most people ran their screen resolution smaller than this at the time, the idea was that the computer operating system would resize it for the smaller crowd, while providing a high quality post for the few who had larger monitors
that has changed in the last couple years, and grown more complicated with wide-screen resolutions that are narrower in height than their predecessors
my suggestion would be to use 1600x1200 .. as this is still a common offering size .. and the people using widescreen resolutions (like, 1600x1024 .. or .. 1900x1200).. can adapt, as they have been .. or, in a special circumstance, ask for a special version ... (that would be rare, IMHO)
I would say, as I have, that a bigger size is better to upload but I disagree that its an average size.
It's hard to get an average size but a standered 17" monitor takes 1280x1024.
Here is a good indicator of the range of sizes and the most common..now I know this is for Steam users but this is a good indicator.
HERE
Search for Primary Display Resolution (1092679 Users)..it's about half way down the page.
You might be able to search the page using ctrl+f or '/' on newer browsers.