The surface speed of the roller needs to correspond to a reasonably achievable surface speed for the bike. If you "gear" the motor too high with too big a roller, you'll burn up the centrifugal clutch and never get all the motor's power to the tire where you can use it.
Weedwhacker motors typically make their maximum power at about 7000 rpm. The surface speed of a 2" roller at 7000 rpm is almost 42 miles per hour. One horsepower from a trimmer engine can't propel you that fast. The motor will bog down and the clutch will burn. The bike will never attain even its max torque speed, let alone the speed at which it makes its rated power.
The surface speed of a 1.25" roller at 7000 rpm is 26mph. That's a realistic top speed for a one-point-something horsepower motor on a smooth running bike with an average sized rider.
Thanks alot guys, is their any other ideas i can make a drive roller out of because i have yet to see a 1.25 bike peg, i have a 1.5 bike peg, also i have heard of people using a socket? any ideas
I have seen knurled axle pegs about 1.25" in diameter, but they were the small kind with 3/8" nut threading in one end. The knurled part was welded on, so there is no reason to think they'd run particularly true, either.