interesting..I can tell you that a breeding pair of angels can be kept in a 20H..they really don't need much room and can defend/tend the eggs much better in this tank.
if you want them in a show tank..a pair can be kept in a 29..my preference is this for a pair. the 29 is taller than the 30 so is more compatible.. angels will get tall. I currently have a large male(2yr old in july) Wild P.Scalare. he is already over 9" tall and growing

you will need to keep an eye on the pair tho..if there is aggression a 29 may not offer enough room.
keep tank mates large enough to not get eaten..mine like neons and Oto's:wall:
there are 3 known species of Angelfish.. Altums, Scalare and leopoldi.
Lupin is correct..the leopoldis are the smallest..they make up for their size with 'attitude' and are the most tenacious of the three.
tha Altum is hands down the largest and IMO the most beautiful of the three..but they can prove difficult to keep. and as of today are still very difficult to breed in captivity...there have only been a few successful breeding attempts.
they also love pond snails..:eek3:
angels are compatible with dwarf Cichlids(rams, apistos etc) you can keep them with larger torpedo body tetras(cardinals and black neons seem to ..uhmmm.. not get eaten as much.)
obviously they will fare better in a larger tank say 40-55G bottom dwellers can include BN's and cories..tho if they lay eggs..the cories may like them too.