This link shows the proper way to measure length:

www.tortoisetrust.org/articles/measuring.htm

basically it is the straight-line distance from the front edge to the back edge.

As for size - in my limited experience (two breeding females) only females 6" or longer have produced eggs.

I've seen a male act interested when he was only 4" in length, but the only successful breeding I've seen was done by a 5.5" male.