All in all, just demo a lot. My tastes tend to be normal for playboats; I suck at playboating, so I lean towards lower-volume. For my weight and skill level, the Jackson Star is perfect for me. Most people I've talked to recommend lower volume for beginners, as they are easier to throw around.
Demoing has just confirmed the above for me. In the medium Rockstar, I feel totally outclassed and find it very awkward. I can manage my rolls and such, but I couldn't begin to throw cleans and other beginning-play maneuvers. When I hopped in someone's star at the pool, it was much more natural to me.
Another thing I've heard is 'get uncomfortable'; like, cram yourself in the smallest boat that you can. I don't think that holds true anymore. Boats are a lot comfier now. I am yet to try a small Jitsu and a small Project X, but they both look very cushy.
Demo, demo demo, be patient with your choice and don't let people tell you what boat won't work for you. Go ahead and try it.