I disagree that anyone is too big for a 125 two stroke. Seat height and suspension they are essentially the same as any other MX bike. They are obviously lacking in power compared to a 250 two stroke or 450 four stroke, which can be a plus or a minus.
I am 6', 200 Lbs and I started off on a 125 two stroke. I really enjoyed that bike and still have it, and will choose to ride it when speed is not an issue. I finally bought a 250 two stroke when I started racing enduros. My best finish (2nd place Super Senior) was on the 125 two stroke, however.
On my last enduro I had the opportunity to ride a 450. This came about because my 250 broke down on Saturday and my 125 broke a clutch cable 30 minutes before the start so a friend loaned me his bike, a WR450F. The only chance I had to ride this bike prior to the race was the 500 yards from camp to the start line, and I almost looped it off the start line. Thank God for the three for free at the start as it gave me time to get used to that low end torque. I don't think I ever opened it up the entire enduro.
My experience is that a quad and bike have very little in common so I wouldn't count on your experience on the quad to help you out much. You are going to want a bike that is easy to learn on and won't get you seriously hurt.
A 125 two stroke is a reasonable bike to learn on. On essentially flat ground it will do fine, more than enough power. You shouldn't have too much problems on moderate hills, you just need to keep your momentum up. It is the gnarly up hills that the 125 will be harder to manage as you have to keep them in the power band to have a shot at climbing the hill. This means keeping your momentum and slipping the clutch when necessary.
Rod