Because the software relies on industry-standard gas equations, it does not allow you to directly override pipe sizes. If you need to specify a pipe size (instead of letting the software automatically choose), you can influence the sizing by iteratively adjusting the pressure at two known points. To do this, insert a regulator valve on each end of the pipe you want to size, then adjust the pressure difference between these two valves until you reach your desired pipe size. Note that even if these regulators don’t physically exist in your real system, they must be included in the software model to achieve the correct sizing.