50mm felt more stable when we first tried them during the big changeover. Might be better for your size. Other than that, most new karts are coming with a 50mm setup.
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.