This has started to bug me more and more, and I don't know if I'm alone in this.... people are starting to post images on these forums at sizes that are too big for my tastes. As I mention in the subject, bigger is not necessarily better.
I have a 21" widescreen monitor running at 1680x1050, which is probably above average when it comes to most web surfers, and probably average for most of the photographers on here.
I find that the perfect size for viewing images on these forums is with an 800x800 px limit. Anything much larger, and I can't view the entire picture without scrolling up and down. I think some people feel you lose IQ when posting at a "web size" and want to post as large as possible, but this is not true at all. IMHO, images look much better at a reasonable size (between 600px to 800px max on either horz or vert.) than at larger sizes.
I would say 1000px in any direction would be max for posting an image embedded in a forum in my experience. I personally limit my images to 800px in either direction.
Just my 2c. I would love to hear other opinions.