My experience
Just thought I'd throw out my experience with a TTR-125L.
For me, it's been perfect. I am 5'2", with 2-3 yrs off-road experience. The TTR has been great because I've been riding the single track here in MI. The trails are tight and sandy, which lead to lots of low speed spills (it's not just me, honest). The little TTR is easy to handle, easy to pick up after falling, and generally best to horse around by hand which is often required here, and on my buddy's place when he decides to take on a cross country tour (straight through the woods). It has plenty of pep there. On the open two-tracks, I could stand for more but it is not slow or boring, ever. I've never taken it to an MX track, but I have heard it won't take much in terms of jumps, etc.
I do feel like I am starting to 'outgrow' the bike in terms of experience and kinda wish I had gotton the Honda CRF150, but the price was hard to beat as I was looking at $2300 for a brand new TTR, or $4500 for the Honda. I have ridden both TTR-225's and a CRF230 and they are just to big for me on the single tracks. On forest roads, they would be fine, but they seem heavy.
I recommend you look into what riding you plan on doing and take our advice based on that. I think it is much easier to learn on a slightly smaller bike than you'll want for rest of time. It has also been my experience that guys that are 5'11" and 200 lbs have little appreciation for what it is like to barely be able to touch the ground or pick up the bike after 3 hrs of riding.
One last suggestion is look at used bikes, but not OLD bikes. A bike in good running condition, good tires, etc, is well worth the price.