On topic: My first recumbent had dual 20"/406 wheels and came with a 63t ring. I replaced that, first with a 60t and then with a 56t.

Off topic: A disc brake makes a really lousy tandem drag brake, as it can't be engaged all the way down a long descent without the likelihood of overheating and loss of function.
If a disc brake is used as an auxiliary rear brake and is used for periods of slowing alternating with cool-down time, then that can work. Our tandem came with V-brakes plus a rear disc. I removed the rear V-brake and set up the disc (with 203mm rotor) as the primary rear brake. If we ever need more stopping power - for loaded mountain touring, etc. - I'll add the V-brake back as the extra brake for use by stoker.