Exhaust pipe size is connected with the engine's size and total displacement, so the higher the engine's output, the larger the pipes will be. Generally, you should match the exhaust diameter to the total engine displacement.
Check the exhaust pipe size chart below to see which exhaust pipe size will be suitable for the size and power of your engine.  Note that pipe size normally refers to the exhaust pipe's outer diameter. Engine displacement (ci) Power (hp) Pipe diameter (in) 150-200 100-200 2.00-2.50 200-250 150-250 2.25-250 250-300 200-300 2.50-3.00 300-350 250-350 3.00-3.50 350-400 300-400 3.50 400-450 400-500 3.50-4.00 450-500 450 4.50 Read more