Like you I got back into dirt bikes as an adult. I had a KX80 & ATC90 along with various go-carts as a kid, but when I was 25 I decided to get a bike after riding with a co-worker who loaned me a bike for a few rides...I was hooked. He suggested a KLX300 (this was in 97) for me, since it'd been about 10-15 years since I'd thrown a leg over a dirt bike. This co-worker had been riding his entire life so I trusted his advice.
I was happy with the bike, but at 6'2" and 210#'s and four years later I realize that I need a bigger bike. The KLX sits a bit lower than other 'non-foo foo' bikes, not to mention I'd like more power now that my riding skill has (I think) increased through out the years. I'm not in a position to get another bike, so the KLX it is, I'll make the best of it.
I guess if I were you, and judging from your larger than average size, I'd go big. At least a 400, mabey check out KTM's 520. The XR600 is a tank of a bike, but has a loyal following and I've seen people use it in tight stuff, but to me it's too much weight.
I think you'd be disappointed with a 250 thumper's power. Although you'll be able to throw it around in the tight stuff, it'd disappoint on a fire road...just my 2 cents