hey just telling you, im 14 and 5'6" and 125 pounds. I bought a crf150f almost a year ago and its a great bike, nice power, but already i want a faster bike. If you have some experience then you should maybe get a 150r. you will fit it. Just telling you though, its a motocross bike and is alot more powerfull than the crf150f. It doesn't have that much top end but still its really strong.
dont get like a klx140. gay
if your going with one of the regular four stroke trails bikes then get a crf-150f. Thats if your getting one.
Look into a crf-150r. If you want somthing cheaper, maybe buy a used 2 stroke 85, but make sure you know about the maintenance and all of the things that come with two strokes
good luck