Which dirt bike is right for me?

[|m| Angel |m|]

I am a 15 year old girl. I am 5' 8" and weigh 104 lbs. I am looking for a cheap ($500-600) used, reliable dirt bike. It will be used mostly for trail riding and a little street riding. The bike does not need to be too powerful, just needs a nice amount of speed. It does not need to be street legal. I am looking for a bike that is not full size, as I have never really ridden one before and do not wish for too much power, but I am afraid that dirt bikes sized for junior riders will be too small for my long legs. Any suggestions?
Would a Honda CRF150F or an XR 125L be any good??I was looking at a Yamaha YZ 125, but it is a motocross bike...