ELI5: How is the length of the runoff/end of the water slide determined?

How is the length of the runoff/end of the water slide determined?

So in my mind there would be an optimum length for the end of a water slide to stop a human.

But then when you add a body of semi stationary water that would add resistance, reducing the distance required to slow a human to a stop.

So what would be the calculation?

The slide that peaked my interest is a single person waterslide sort of like a spiral with a 10m horizontal section at the end for riders to alight from the slide.

Wish I could attach a picture of it.