The X-Large is 51cm. Which is a 20 inch bike. A jumbo is 57cm. Which is 22 inches. A 20 inch bike should fit a person that is around 5'10-6'1 and a 22 inch should fit a person that is around 6'1-6'3. Do not get one that is smaller than a X-Large. If it was me, I would get a jumbo (22 inch). I'm 5'10 and I ride a 19 inch. He is 6'1 (which is 3 inches taller than me). A 22 ich is 3 inches taller than a 19 inch.
http://www.cannondale.com/2012-quick-4-21002
Click on Geo to see the sizing.