To get the bow length, use a flexible take (like a steamstress' tape--a cloth or plastic tape) and start at the "top" of one of the string grooves. Measure along the side of the limb, following the contours of the limb, through the center of the riser, down the other limb to the "top" of the other limb tip.
Most bows will get a string 3" to 4" shorter than this, but it will depend on the bow, if the bowyer followed AMO specs, etc. I've seen string lengths run from 1.5" shorter to 6" shorter.
The same string will probably work on the Bear and the Shakespeare--most of those get a string about 4" shorter.
Like Vermonster said, according to AMO specs the AMO bow length is 3" longer than the string that puts it at the proper brace height. The "catch" is a lot of bows don't follow AMO specs.
Chad