I'd suggest wearing a helmet and goggles. At 6'5", I would assume a portion of your head is above the windshield. That means lots of bugs plastered on your forehead. It won't be long before your forehead carries the same scars as your front bumper!

I'd say that the length of your torso is the determining factor, not really the height. Based on the height of people I've had as passengers, I'd say 6'2" is probably the comfort limit. Although there are probably many who have given up the comfort level because "they had to have one".