You're actually the typical height/weight for what "stock" suspensions are set up for. Theoretically, you could hop on any machine with very little tuning needed to have it set up for you. It'd basically be tuning for your particular feel.
As for actual size, go to the dealer and see what you like. See what feels good to you, from a perspective of being on the seat. You probably won't want to go out and get a brand new 450 4 stroke, since that's about as powerful as you can get right now, but if you don't plan on upgrading in a year or two, that might be your only option.
Otherwise, look around for a used cheapo 250, get some mileage on something you won't feel guilty about dumping all the time, then start looking bigger for next season.