whoever told you that a 125 is too big for you is really stupid. i'm 5'7 and i ride a 250. its only a tad big for me. ride a 150 or 200 unless your a skinny f,ck but you sound like your at the right bmi.
btw, you put dirtbike question in a cycling category. so i dont think you'll get alot of answers. hahaha