What would be the recommended seat height for someone who is approx 6 foot 5 inches tall?

A taller rider can manage just about any machine on the market and even ride a pocket bike with their knees around their ears, but your main concern for the road is going to be a comfortable reach to the pegs and bars. Triumphs and Buells are quite tall bikes so have a look at them. The best way for anyone of a non-average size to find a bike that fits them is to get to the dealerships and sit on a few machines until they find one that is suitable. If you heart is set on a certain bike then you'll need to look into ways of making it fit you, perhaps with wider bars, adjusting the position of the pegs and extending the seat back to get a comfortable riding position. You can make most bikes a little taller with longer or extended rear shocks and by moving the forks down in the head stock. Doc