The biggest that the space and you can manage. Our 2 acre home back
yard has a 4,000 gal system that fits where it is located by the deck
and behind the house. If I could do it again, I could not change
much. Our aesthetics for the width of the deck and the length of the
house dedtermined the size. The size is determined by the location.

Our 35 acre property is another story! The pond there is an acre.
The size was determined by the most that the runoff would support. It
is dirt bottomed and would be hard to work for koi. It has bluegill
and bass.

Part B of the issue has to do with the fish you are running. The rule
of thumb for koi is 1,000 gal for the first and 100 per additional
koi.

Your pool has more depth than you need, but not more tha you can use.
It could be beautiful to have 6-8' of depth for your fish. If I were
building it from scratch, I would probably make it shallower for
cost's sake.

Have you ever been to the Japanese Tea Garden in San Francisco? When
I was htere in the 1960's, it had a great stream setup with fish.
Huge and winding.

What is the biggest pond that will fit your aesthetics? That is the
ideal size. IMHO

Jim