It is my understanding that a 3" bottom drain is effective at sweeping a 6' to 8' diameter and pushing the limits to a 10' diameter, depending on who you talk to. Of course, if you are going to push to the limits, an air dome is highly recommended for effective sweeping. Where as a 4" BD would effectively sweep a 10' to 12' diameter for optimal sweeping and a 14' diameter would be pushing the limits.
Now mind you, we are assuming the bottom of the pond should always be bowled down to the lowest depth of the pond where the BD would be installed for the most effective sweeping of debris for removal from the pond.
I suppose this information is all subjective depending on what the MINIMUM, OPTIMUM & MAXIMUM flow rates are being used, what you are flowing into, how heavy your fish load is, how much leaf debris you pond is subjected too, if you are raising male or female koi, and again of course who you are talking to.
But what do I know? Look what happens if I stay up to late on the computer
With All Due Respect,
The Pond Digger
P.S.
I'll go pick out a switch..............