I personally think you will have more fun on a 2-stroke 85cc or what I would suggest, a 100cc. I would suggest a KX-100. Its a 100cc 2-stroke. Vrs an offroad 4-stroke like an XR, a 2-stroke 100 is great, will have a lot of power, are cheaper, easier to maintain, have better suspension, are MUCH MUCH lighter than the XR, and have a wicked powerband that will get you out of any sticky situation :)
While 2-strokes might require slightly more maintinace, they are easier to maintian. Besides having to premix the oil, your not going to have to worry about anything that major besides cleaning the air filter and chaning the transmission oil every couple of hours. Kids around here dont redo their top ends until 125+ hours, never even look at their air filters, dont clean the bikes at all, and put any old oil and premix ratio they feel like in some of their 2-strokes down here, and they are still running. They are tough bikes. And if a 2-stroke ever does break, or when its time for a top end job, a 2-stroke is easy to work on, while with a larger bore 4-stroke, you need to have a mechanics knowledge to begin the work, or atleast be pretty darn knowledgable about automechanics.
I am 15 yrs old, 5'5/5'6, 115 lbs, and I ride a KX-125. A 125 will deffinetly be a bit much for a first bike, and will seem a little intimidating, so a 100 will probably be an excellent choice. Good luck and let us know what happens :)
And have fun riding in all that land!