It doesn't matter how big you are its how you play the game. I play rugby and I'm small but I get in there and get trys.
I think for a forward, weigh anything above 100kg. For a lock, it's preferred that you are around 1.90 or taller i'd say. For a back, anything above 80 is the reasonable weight. Tbf, you can be lighter, but most players lighter are the scrum-halves who are usually short too. You can be any weight to play in the backs. But it's better the heavier you are for a forward :)
I would say they would perfectly weigh about 14 stone, but this can't be because they are fat, obviously or they would not really be able to run. Obviously it depends on their position but I am talking in general. I would say height wise 6 foot is good, not to tall and not too short.
