My son is 6'5 and 300-310. He's happy as a clam on his Motobecane 29er from Bikesdirect - a 29" mountain bike is a great way to go for big, tall guys, IMO.

For tires, I've put the Schwalbe Big Apple 2.35" semi-slick tires on his bike, and they offer a great ride. Great tire. http://www.biketiresdirect.com/produ...-apple-29-inch

A good saddle will also help. Don't be fooled by the super-cushy, super-wide saddles - they can actually be LESS comfortable and can present more circulation issues than a good quality saddle that is properly sized for the person's sit bones.