It is around 1 3/4" in diameter. Also, bigger is not always better, depends on what your system is. On SC cars you will loose bottom end torque in exchange for dyno bragging horsepower. On a 11 PSI car we did back to back test with the 2 1/4" FM midpipe verses stock and saw 6 ft. lbs. less torque and 5 HP more with the larger midpipe. Torque is what you feel!
So suggest don't going to big. If I were making a normally aspirated exhaust it wiuld not be any bigger than 2 1/8".
Good luck.
Gary