I think 500 pixels is big enough, I don't recall 
what resolution I use to scan my cels- I used to 
do it too high and then I realized the images 
could look very good at large sizes with much 
lower resolution- my large scans are 750 across, 
but I put small thumbnails (200 to 250) so the 
page loads faster.  I am a big fan of thumbnails, 
REAL ones, not trying to resize an image with 
html, which makes it take as long to load as a 
large image. ^_^