How big

First off, welcome to the boards bro. :cool:
check this link out.

http://www.dirtrider.net/forums3/showthread.php?t=108916

You should really fill this out to get the most help. But in any case, an xr 650 (trail oriented) or 250 2t or 450 4t (mx oriented) will be generally what you should look at given your size and experience with other bikes. They all have their ups and downs. You have to decide what type of ride fits you best, a 4 stroke or 2 stroke. They are completely different animals but can be equally as fast. Just be aware the biggest differences between an mx bike and an enduro/trail bike. The mx will be more powerful and less weight per cc but a lot more maintenance with a rougher ride on trails out of the box. The trail bike is designed to be a bullet proof tank with softer suspension and other commodities such as electric start, brush guards, bigger tank, etc. But, they do weigh 30-50 lbs more depending on the model and arent made for bigger jumps due to the suspension, weight, and distribution of that weight. The great thing about dirtbikes are the options you have when selecting one. I personally, am striving to have one of each type one day. Hope this helps and good luck.

An 02 to 04 250 2 stroke Would suit you nicely. A bike like that will fit your price range and has more than enough balls to carry you.

Most people would say a 125 is too small for you. I believe they are geared more towards 140 - 160 lbs. riders but I love mine and weigh 185. If you were close I would let you try both of mine. There is a big difference not only in the power, but the delivery of that power. The 250 comes alive much quicker and has more torque. I wouldn't cancel out a Honda XR 400 - 650 either if you can find a cheaper one. The best advice is to ride em all before your dead set on one type.

The YZ-125 or CR125 will handle your size nicely and can be a blast to ride as they are light and nimble. On flat ground and wide smooth trails you won't have any problems.

When it comes to climbing hills your weight is going to be an issue. The 125 will do it IF you keep the RPMs up. Hills will not be easy for a beginner.
You will NOT want to enter an enduro on a 125 two stroke!

A 250 two stroke will suit you much better in the long run, as long as you don't kill yourself in the first few outings. A two stroke has a very "peaky" power delivery that can be difficult to master and it can get you into trouble when the power hits when you weren't ready.

Rod

I have an older RM 125. I am 6' 185 pounds and the bike fits me well. I have a friend that is inexperienced and about 240. He has ridden the bike twice and says that he can already tell he will outgrow it quickly. If you are sold on a 2 stroke, I would go with the 250cc. If not, an older XR350 to XR600 might work for you. I had an old XR500 and I loved the torque. Those bikes have much more managable power for a beginner than a zippy 2 stoke.