If you're 90kgs now, you'll probably be a bit leaner than that after a few weeks of pre-season training.
You're either going to be a smallish back rower or a small to average sized centre.
Other option if you have a decent passing game is that you can slot into Hooker. A high work rate never goes amiss there.
Back row, you'll probably slot straight in without having to learn too much. Centre, you'll need to learn a bit about your spacing, defending numbers, when to come in or stay out.
Hooker, you'll have to learn heaps. It's one of the most technical positions in the game.
Basically... It comes down to how much you'd like to have to learn.