The content area of normal pages is 695px wide, and of course variable length. The right sidebar button width is 260px, and the internal content area varies a little, but max should be about 240px if on white block background, or 260px if transparent (like the quotes and social networking displays).
On pages with no right sidebar, the content area is 955px wide.
The left sidebar is available but not populated at the present.
Images that are off-size can reduce effectiveness of our site, so here are the actual image window sizes for the dynamic areas on the front page:
Dynamic Display Block main image panel: 447px wide by 300px high
"Pager" images (the row below the main dynamic image panel): 70px wide by 50px high
Feature Blocks 1 & 2: 75px wide by 170px high
When an image is way to big, it takes noticably more time to load it. One image we had recently was so large that it took a couple of seconds to fully display... in the PAGER image area. Use Paint.Net, and edit the image. Crop and resize it to fit the actual image space.
If the image is too small, try to find a better one to use instead. A 16x16 image might work fine for an icon, but it's not an ideal image on the big screen.