Well firstly, Pipe diameter are depends on Flowrate, Velocity, Compressibility, temperature and Pressure. Here is the equation/formula that i ussualy use for pipe sizing calculation. But the most important thing is that you have to design the velocity. (velocity limitation : below acceptable noise levels, it's about 60 to 80 ft/sec and if the gas contains liquids we have to refer to API RP 14 E which is the minimum velocity should be limited to about 10 ft/sec).


V = 1440 * Z * Q * T / ( d * d * P )
Where V =velocity ft/sec
Z = compressibility (ussualy 0.94)
Q = Flowrate, million cu. ft/hr
T = temperature, R
d = inside diameter of pipe, inch
P = operating pressure, Psia

Good luck ^_^