I think @Fish Folk just won the internet for today, but I can add one thing! I have 1 angel in a 29 gallon, and it's really pushing the edge of being big enough for him.
For what it's worth, a 20 gal might have been big enough when he lived alone. He used to pick a parking spot, in any size tank, and just sit there. But now that he has platies to chase, he'd probably love a full 6 feet. So if you want a single angel with no stimuli, then maybe.