Depends on where you start your measuring. This gets into a little more philosophy of bow building than you need right now but I argue that the limb starts at the fulcrum. The fulcrum is the center of the bow so the limbs are equal lengths by default.
Other seem to argue that the limb starts at the fade. This separates the handle from the limbs, at least in thought. In essence, it seems that there is a top limb, a handle, and a bottom limb.
I prefer to think of it as a whole so separating a bow into its parts, even into upper and lower limbs, is awkward for me. That's why I like the fulcrum tillering method.
For more philosophy, later on read Dean Torges' "Hunting the Osage Bow" and his writings at bowyersedge.com.
But, for now, just build a bow.Then build another one, and another until you figure some stuff out for yourself.Less thinking more working. Here's a working man's board bow tute:
http://sticknstring.webs.com/ferretsboardbow.htm