If you are going to have koi then you should have maybe about two to three thousand Imperial Gallons of water at least, depending on how many koi you wish to keep as well. You could start at 1000 Gallons and then add on another connecting pond later in the future. My ponds have changed so much in the last 20 years and no matter what you do , there will always be something you would do next time, like build it much bigger, most people regret not going deep enough or large enough.
I myself like the three connecting ponds, now all together about 3500 Imperial Gallons, which are bigger than US Gallons by the way. The top has the largest and biggest fish as they need freshest and most oxygen of all the fish, then it flows down to a middle pond that has a large yellow flag as a floating natural marsh, which is also not planted in any soil.
Then it flows down to the bottom pond where all the smallest fish end up. I have water gates that prevent the large fish from going down stream, only the small fish can get threw. The bottom pond is usually the dirtiest pond as all the leaf litter and stuff falls into pond, so I have the big yellow flag right at the edge of the middle pond, just before the water dumps into the bottom pond, that way it catches all the stuff and "eats it"
Way back in the early 1990's the now top pond was then the bottom pond, and I really hated the stepping stones right next to pond, cause would chips would fall in and the lime would leach out of the cement and find its way into the pond after a heavy rain.
then I build up the ponds and reversed the water flow direction and this was a side shot of the then lower little bottom ponds.
Originally it was just one pond, see red bench is the same... I think I changed that again the first summer...
That was looking to the back, notice the small fence and red bench have not changed...
Winter view. Ponds got much bigger and more build up. Then I changed them several more times till I ended up with this (see red bench and small weathered fence the same.
Summer look I changed the walk way from wood chips to cobbled. Lots of the round stepping stones are still in the same place they were over 20 years ago
From Poppy's View as she is walking by the now "bottom pond"