liter per second

Litres per second is the metric working unit for anything you can put a hand in front of. A kitchen tap at full flow is around 0.15 L/s, a shower head 0.1 to 0.2 L/s, a residential service line perhaps 1 L/s, and a mid-size building circulating pump 5 to 30 L/s. Multiply by 3.6 to get cubic metres per hour, the unit most European pump curves are drawn in, and by 60 to get litres per minute, which is how flowmeters and hose fittings are usually rated.

Flow rate connects to pipe sizing through the continuity relation $Q = vA$, where $v$ is the mean velocity and $A$ the cross-sectional area. Design practice keeps water velocity in the range of roughly 1 to 2.5 m/s: too slow and solids settle and pipes are oversized, too fast and pressure loss climbs with the square of velocity while erosion and noise become problems. A DN50 pipe, about 50 mm bore and 0.00196 m² in area, carries about 3 L/s at 1.5 m/s, which is the kind of arithmetic that sizes a system in one line.