Dirt bike for 150 lb beginner?

I am wanting to get a dirt bike. Ive rode them before a little bit but not much. I am 150 pounds 5 foot 11 inches. I want to know what size I should be getting. I will be riding with very experienced friends that are my size riding 450's and big bikes. They all have yamahas. I kinda want something different. What size would fit me best? I want to keep up with them and not have a puny little bike. Any suggestions?