It all depends on where you feel more comfortable. I would try to find some trials riders local to you with mod and stock biked that you can try out. I am just shy of 6'1" and weigh in at a hefty 225lbs, I ride both mod and stock, and enjoy both. I have an old Monty x-stam, which is shorter than newer bikes, but I can still easily ride it. Get out and try some bikes, it has to be what you want to ride, not what others tell you that you should be riding, if you are going to enjoy the sport.