Wider pipes transmit fluid at higher rates, with all other factors remaining constant. The wider pipe contains a larger volume of water in any given length of pipe. A required volumetric flow rate therefore corresponds with needed pipe width. Other factors also affect volumetric flow, and you must take them into account when calculating the pipe's width from the flow rate. These factors include the length of the pipe and the viscosity of the fluid.
Multiply the fluid's viscosity by 8. Water, for instance, has a viscosity of 0.01 Poise: 0.01 x 8 = 0.08.
Multiply the distance over which the fluid must travel, which forms the pipe's length. With a distance, for instance, of 300 centimetres: 0.08 x 300 = 24.
Multiply the flow rate, measured in cubic centimetres per second, by this answer. If you must move, for instance, 400,000 cubic centimetres each second: 400,000 x 24 = 9,600,000.
Divide the answer by the pressure drop across the pipe. With a pressure drop, for instance, of 250 dynes per cubic centimetre: 9,600,000 / 250 = 38,400.
Divide the answer by pi: 38,400 / 3.142 = 12,221.5.
Raise your answer to the power of 0.25: 12,221.5 ^ 0.25 = approximately 10.5 centimetres. You need a pipe with a 10.5 centimetre radius.