I am 5'11", 110LB and have a 1997 RM125. Bought it for a grand, and have since had to put some new parts in it..All new bearings, new rod, and a new piston. So, for about $1350, I have a very good shape late 90's RM125.
The 125 has more than enough power to throw me around, but you are just a bit heavier than me, so you would probably want to look into a 250. A friend of mine is around 6'3" 170LB and rides a RM250 (Unsure on year), and he has a blast on it.
I'd say try to find either a 125 or 250 and see if you can give it a test run. If you feel that the 125 doesnt have enough power, then go for the 250.
Also, unless you are prepared to spend a lot of time and money on upkeep, go for a 2-stroke. 4-strokes are much more expensive to buy, and much more expensive to maintain.