Don't believe the hype with all that gotta be big to be a back. Yep Height helps, but being wider than a Dorito does not. No offence to the big guy's but having 3 portly backs can seriously shoot you in the foot. Sometimes it can be better to play a front at the back or develop a small back player. In this instance your losing (or should be) your safe cosy back bunker. Learn to move, and be prepared to play on the 30's and 40's if you have to.
Getting it in your head your just a back player is gonna ensure you never develope your game fully.
By the way I used to be a right chunky monkey, lost some weight and played the 50, I'm now 5ft 6, 10 stone play at the back and carry 14 pots into a game (dumped 10 in one game at last m25). So size don't neccesarily mean sh*t.
Oh and check out da Huntingdoen beach DVD