I also prefer 1000 as a width.
As an experiment, I am posting here three versions of the same jpeg, one each at 800, 1000, and 1200 px width.
The thumbnails are all the same size, and the 800 and 1000 pixel versions fit nicely on my screen when clicked, but the 1200 pixel version does not fit on my screen so I must either scroll to see the whole image or else zoom my browser smaller so I can see it all, but then I can't read any text.
Thus, I would much prefer a limit of 1000 pixels, with any larger image displayed on its own separate page.
-Eric
P.S. 1500 pixel version added for Slobodan.