Here's my cheat sheet. All log diameters are approximate, not nearly as accurate as Jim's method. Also, this does not account for sap wood, I honestly didn't have a problem with it. However, some that I let air dry for awhile did turn blue on the corners. Others, that were cut then immediately planed and kept out of the wheather did not turn color. I made my timber list first as Jim described.

6x7: 10" log
6x9: 12" log
8x8: 12" log
8x10: 14" log
8x12: 16" log
9x13: 20" log

Of course this is measuring the smaller end of the log. Good luck with everything!