Mini Hydro Power Project

Infinitely large.

Now, on a more practical issue, the larger the better for reducing friction. Consider the cost of pipe and the value of lost head -- there should be a crossing point of the two curves, so that may be the best compromise.

Please check your's units . 24 m3 / sec is not a mini-hydro

a 3000 mm diameter will give you a 3.39 m/sec speed

The first power plant at my city was on a river about 12 m3/sec