I am about 230lbs now and on a weight/diet plan so could conceivably get down to 200 minimum. I am 6'4 so anything under 200 and I look like a lollipop. I know the suspension would need to be redone.
CR250 seems to be a bit much for me at this point and thought it might benefit me to go to the 125 until I become better accustomed to two strokes. Physically, the bikes are about the same size so it would really come down to whether it will pull my big head over the big jumps.
I weigh 170lbs and hate 125's. At your size you will have to ride a 125 like you hate it just to make it go. I also recomend the KTM300 or 380. You will just have to go easy until you get familiar with the power. Good Luck