I'm very similar to yourself and I play second row but I'm 5ft.11 an 11 stone (which is still quite light for a forward) but your height should be fine. If you don't want to be a forward then try out centre, winger or full back. If you like hitting hard and getting really stuck in to the game - Second row If you still want to hit hard but don't want to be a forward. If you can execute moves well - 1st centre If you are very fast - Winger If you want to watch the game and see where to make you next move. If you like catching - Fullback I hope this has been helpful.
flanker
We need more details. Right now you could make a good flanker or number 8, but also a good winger. Due to your height and just "ok" passing ability, along with your slimness and quickness, I'd say try out for flanker or number 8. To be a back these days I'd say you need to have the perfect passing to go with your speed, or you need to be wide and strong, not slim.
