One thing to remember its you're usually wrapping over the part of the tail material that's tied to the hook shank. Some materials tie in thicker than others. For example, if you tie in a marabou tail, you end up with a fatter body than you would with a hackle tail. Also, some types of chenille compress more than others. That means you have to make more wraps to completely cover the shank. Braided chenille and crystal antron chenille don't compress down much, while candy chenille and glimmer chenille have almost no bulk and are pretty much nothing but thread.
Most types of chenille are pretty cheap. IMHO, it's best to buy a 3 or 5 yard pack for around a dollar and try out several sizes in each of the style you like.