The only reason you should go to a 50mm is if you can’t get a soft enough axle in 40mm. At the sticky euro tracks they were breaking/bending 40mm axles that were too soft so they went to 50mm. I’m 6’1″ 200 Lbs. and I run a medium 40mm on an old (06) Margay.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience