It depends more on the water depth, angels being tall fish. For the casual fishkeeper a 20 is pretty tight, I wouldn't go with anything less than a 29, you get a couple more inches of water depth, and a bit more capacity for tankmates. Traditionally angel breeders have been using 20h tanks for pairs for many years, 16" of water depth with no substrate. I've been using 29's for pairs, that extra depth helps, especially with veils.
A 20h with substrate cuts that water depth, you end up with ventrals trailing on the bottom, leaving an opening for infection. You've got 4, maybe 6 months until a deeper tank is needed, I'd guess a properly raised angel at that size to be 8-12 weeks old.