What about the asin tag? That seems to default to the larger pic (500x500?), whereas I think 300x300 would be fine.
The Amazon ASIN tag is resized to 400px across.
Image size is relative to the screen resolution you are watching. I'm on a Mac with a 27 inch screen with 2560x1440 pixels. A 400 pixel CD cover takes up just 15% of my screen's horizontal resolution. Anything smaller, and I would have trouble seeing it. (I'm getting old)
Granted, not everyone is one such big screens, and therefore the image will take up a bigger proportion of the screen. I think 400px is a good compromise for everyone. I'm happy to hear otherwise.
In the meantime, for PC users - it might be worth checking that you have your optimal screen resolution for your monitor. Often, the default setting is too small. Check
this page for instruction on how to change your resolution.
cheers
Rob