What size mountain bike frame for a 6 ft guy and 250 lb..i am not a fat *** i have verey powerful legs ?

You should go get measured at the bike shop. Large would be a good starting point, but depending on bike mfg, riding style, and your proportions (leg lenght, arm lenght, torso length, etc) you might be on a different size.

The pros at the shop will also be able to show you how to have your seat and handlebars adjusted to optimal position.

If you are looking to avoid a new bike and buy used, I can tell you that 90% of 6ft tall riders will fit a large reasonably well...but again this is just general terms.

I will say that if you are truly lean and strong, you need to be careful of what kind of bike you choose. A bike that might be suitable for a 165# guy might bend and flex like crazy under a Clydesdale sized guy. My $.02 is to avoid exotic materials like carbon, titanium, etc. Stick with steel or aluminum.

Go to a bike shop and let them help you. They will fit you properly. There is a lot more to it than just your height and weight. They can also help you select materials. Where and how you want to ride is an important factor in selecting the proper bike for you. Once you have all this information, you can make apples-to-apples comparisons with other shops. Don't feel obligated to buy a bike from the first shop that fits you. If they don't offer the best value proposition, then they do not deserve your business. Besides, you will be back to buy tires, tubes, accessories, and clothing. I've heard they make more money on that stuff than they do on the bikes.

HTH

Well lets see here, I am about 6 ft and ride a 19 inch frame. I only weigh 170 but that shouldn't make much of a difference as far as frame size goes.