Im new to Dirt Biking, what size bike should I buy?

I think a 250 2 stroke would be great for you. I am a Yamaha guy, especially when it comes to 2 strokes, so I say check out the YZ250. The Suzuki RMs, Honda CRs, and Kawasaki KXs are nice bikes as well, I just prefer the YZs over them. If you don't want a 2 stroke, check out the 450 4 strokes, like the YZ450F, CRF450R, etc. Lastly, if you think these bikes as a bit much, check out the 125 2 strokes and 250 4 strokes, such as the YZ125 and YZ250F. I think once you take a YZ250 for a ride, you will not want to get off it, but as we know all people have difference preferences. Good luck with whatever you go with!