This link has some good info on the aspects of buying a board and the differing types you get.
http://secure.session1.com/index.php?cPath=253_402
I am a beginner at this so anyone should feel free to correct me. Most advice given to me seems to be around a 140 to 145 x 40ish (I’m around the 15 stone mark) would be good to learn on and allows for some progression. I would think a twin tip around the 140x40 mark would be good starting place. I’ve been told that larger than this and you will outgrow it unless you plan to get 2 boards (a small one higher wind days).
People also say get out and try different boards if you can, I’m sure there are some people who would be willing to let you have a shot and see how you get on. I have just got a cheap 130 (cant remember the width) gastra mutant board (courtesy of Dave getting a new board

) ) Probably a little bit small to start on but can’t complain too much as it was cheap. More than happy to let you have a shot.
I’m sure some of the more experienced kite surfers will be able to advise with more info.