This size works well with Netscape's default window size. Larger images will not display completely on most monitors. For example, a 2 X 2 inch image with a resolution of 150 dpi will be about 4 X 4 inches on a 72 dpi monitor. Try to keep graphics files smaller than 30K. Large files take a long time to load, especially over telephone lines. Keep text simple and large enough to read, especially if you intend to project your pages via LCD-panel during class.
Illegible pages are useless pages.
Ideally, they reinforce text-based information but do not merely repeat the information. Use common sense. Highly textured backgrounds, while distinctive, are tiring to read from. The same goes for bright colors. They make an impression, but you probably do not want to present 15 pages of required reading for your class on a page that makes your eyes bleed.