Tank size for the pair doesn't matter, they will spawn in any size tank they are comfortable in. The important part with size is filtration, as you will need to feed to saturation for 2-3 times a day and a small tank unless it is part of a much larger system simply can not handle the bio load.
As for larva rearing a 20g tank is a disaster in the making. Smaller is better, 5-10g. If the tank is too big, you will have a very hard time maintaining a proper rotifer density, not too metion it is a big waste of water, algae and rotifers. It's like trying to drive a carpet nail with a 20lbs sledge hammer.