Am I too big to play 8-man?

Go look up a man called Pierre Spies and then ask the question again. You are a bit light to be 8th man (unless you are still very young, but at that height I doubt it)

The key to 8th man is explosiveness, acceleration off the mark, ability to hold the ball in the tackle and crash ball. Most of the great 8th man players started their careers either as lock in a 12 man game (primary school level) or some very rarely as centers before putting on some extra mass.

Most of the 8th men in the Super Rugby championship are your size.

You're not too big to play 8 man no. But I think you'd have to compete with the current 8 man on your team. You're going to have to ask your coach really, show him why he should move this perfectly good lock to an 8 man. I remember when I was a flanker and I really wanted to play Fullback, except the coach never wanted me to do it, I knew I had to work on my kicking and ball catching, then one day he gave me my chance and now I am the teams proud fullback. All I can say is try your best to prove to the coach that you can play 8 man. When the chance arises, take advantage and don't disappoint.