i agree with auburn,
From my own personal experience in motocross, your just the right size for a 250cc 4 stroke. a 125 is the right size, but the engine is designed for beginners who weigh in at around 100 - 125 pounds, you will just bog, unless you can keep on the gas the whole time on the track to keep up your momentum. However, you can bore out a 125cc to a 150cc motor, these are almost as fast as 250s but still a 250cc has calm and smooth power, not like a 2-stoke 125 were is choppy.
If its trail riding you looking for, then yeah a 125 is perfectly fine! your not looking to score the fastest lap time on the trails so... get what ever you want in this situation.
what ever your decision... have fun with it!