This is my desktop, it is a screen resolution of 1680 x 1050 (shown proportionally as 1000px x 652px here).
An industry bible for these sorts of statistics can be found here:
http://www.w3schools.com/browsers/browsers_display.asp - I'd be very surprised if screen size by most members is super large.
I generally post (poster) images that are 530px x 800px or thereabouts (vice versa for landscape) which means on my screen an image takes up just under one third the width and about 80% of the height as you can see by the screenshot- I feel it should be as much about the download time of the page (bandwidth) as the size, a lot of people save their images at too high a resolution (when posting them here) when the reality is there is no need to, 100kb or less is perfectly fine.
The standard size for the two biggest poster image posters (for a one sheet): Bruce's image before the SUPERsize image is 550px × 823px; whereas Heritage's is 450px × 674px BOTH usually 100kb or less