zelph wrote: What dimensions and weight should one be?
That sort of depends on the wool and the diameter of the yarn you want, though there's a wide range of sizes that will work, maybe just not as fast a perfectly sized one. I'd start with a top-whorl spindle thats maybe 3-4" in diameter and not too heavy. At first you will probably have trouble coordinating the twist and the stretch, so too heavy a spindle will break off the yarn more often.
At first, until you get the hang of it, try the "park and draft" method. Basically, you over twist the yarn that's already made, then park the spindle under your thigh or between your knees while you draft out the fibers and let some of the twist run into the roving you have drafted out. It's much easier than trying to coordinate a spinning spindle with your drafting efforts.