Alrighty There People!

I'm writing a novel that is aimed at teenage lads, around the 13-16 year old age mark, and it's an action/adventure kind of book.

Anyway, I was just wondering, how many chapters should such a book have, and how long should the chapters be?

Also, how much should go into desribing the characters physically, bearing in mind the attention span of the reader?

Your help much appreciated.