I figured out the aspect ration on the pix they included in the Reader, and then applied that ratio to the 600 pixel width -- viola: 768 pixels.
I could be off (I
did do the arithmatic with a calculator, so that eliminates a big margin of error right there: my arithmatic skills, or lack thereof), but the pix I made to that size worked well, with no black border anywhere, so not by much.
I'd call 600X770 close enough for jazz too -- the pixels are about 150 micons after all. I figure one or two won't make that much difference either way.