Oh forgot to add. My wife is 4'10 and her bike frame is a 15 inch and it's a perfect fit for her.

15 Ladies fits most riders 4'10" to 5'5"
17 Ladies fits most riders 5'5" to 5'9"
15" Mens fits most riders 5' 3" to 5' 7"
17" Mens fits most riders 5' 7" to 5' 10"
19" Mens fits most riders 5'9" to 6'
21" Mens fits most riders 6' to 6’4”

The two bike that I've recommended was for your price range and for your style of riding. Which you said it would be in dirt roads. Plus the shocks help absorb bump. Which most dirt road might have a few here and there. Unless i would recommend this one. Mostly for roads and hard pavement.

http://www.bikesdirect.com/products/.../avenue_fb.htm