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Pipe diameter follows from continuity: area is flow divided by design velocity, and diameter from the area. Design velocity is kept at 0.6-1.5 m/s — fast enough to avoid sedimentation, slow enough to limit head loss and surge. Head loss is then checked with the Hazen-Williams equation.
Diameter from velocity
Area = Q / v; diameter = sqrt(4 A / pi)
Hazen-Williams
S = 10.67 Q^1.852 / (C^1.852 D^4.87)
Too slow allows sediment and biofilm to accumulate and lengthens water age; too fast increases head loss with the square of velocity, raises pumping cost and worsens surge pressures on valve closure.
New ductile iron or PVC is 130-150, but unlined iron degrades to 60-100 with tuberculation. Design should use an aged value appropriate to the pipe material and expected life.