Add 2 more zeros onto that 20 and that's the minimum a koi could live happily in. We're talking about a 1-2' fish. I'm sure you've seen a koi before, did you not notice that they're generally don't physically fit into a 24-30" long tank?
Your best option at this point is either to build a koi pond or find a nice pond where the koi will be happy. You could post an ad on craigslist or ask around at any nurseries or public gardens with koi ponds.
If you still want to pursue FANCY (no commons, shubunkins, etc) goldfish, you are going to want a minimum of 40g. The rule of thumb is 20g for the first fish and 10g for each additional fish. Goldfish (and koi) are social creatures and it is even legally considered animal cruelty in some countries to keep them all by themselves so yes, you will need 40g at minimum.
Good luck sorting out the koi issue.