If you mean the common terms like "nine inch rear axle", the measurement is the diameter of the ring gear. In general, the larger the ring gear the stronger the axle. GMs typically go by the number of bolts in the rear cover, but the same applies -- a 10 bolt has a smaller diameter ring gear (7-5/8") than a 12 bolt (8-7/8").
That HR link has the best info, but missed the AMC axles. AMCs use a Model 15 (AMC 15) and Model 20 (AMC 20). The AMC 15 has a 7-9/16" ring gear, the AMC 20 and 8-7/8". The 15 has a plug in the rear cover, the 20 doesn't. That makes them pretty easy to tell apart. All AMC axles (with the exception of some Jeep models) have a two piece hub and axle with a large nut on the end holding the hub on, similar to a front wheel drive car. That's their main weakness! That nut has to be torqued to 250-300 lb/ft or the hub could spin on the axle. The key really doesn't matter, that's just to keep the hub in alignment when reinstalling after it's been removed for bearing replacement. There is no re-torque interval, but AMc never intended the cars to last more than 10 years. The last AMC axle was made around 1988, the last year of the Eagle and big Jeeps, and a few were used on some Commanche trucks. The center section and differential, however, are still produced by AM General for the military Hummer -- it's that tough!!