Hey Don, you mentioned from that diameter, but didn't give any. However you should be able to figure it from the link JKJ supplied. But since you don't want to use any sapwood, you should use the diameter for just the heartwood on the small end of the log. Also, you'll loose a fair amount from the juvenile wood around the pith.
As for rule of thumb, being the owner of my own mill, I wouldn't bother with anything less than 18" on the small end. You may find that its a lot cheaper to just buy the lumber you need rather than finding someone to handle the logs, another to saw, and another to dry the lumber if you value your time.