You would want a 125 or 250 like a...
Yamaha YZ 125 or 250
Suzuki RM 125 or 250
Kawasaki KX 125 or 250
Honda CR 125 or 250
Honesty going by your size, you might want a 250, but a 125 will do good to. Thats assuming you want 2 stroke.
Companys now make really nice 4 strokes, in 250 and 450. They are a little too pricey for me (around $3,000-5,000)
A 1998 125 2 stroke from the companys above can be found for around $600-1000 in good shape. Might be able to find even newer for the same prices.
JUST noticed you said u wanted a CRF. If you got the money, a Honda CRF250R is a great bike, and will be more then enought for you. You would be a perfect fit on that bike.