Water depth isn't a big deal for bichirs (12" would work), so just shoot for a large footprint (48" x 24" at minimum with 72" x 24" being preferred) and tons of filtration. Honestly, you could get away with a 70 gallon wide (48" x 24" x 13" tall) if your filtration is good enough although I'd say a 100 gallon wide (72" x 24" x 13" tall) would be far more preferable. Just something to keep in mind in case you want to save a little money on the aquarium in order to put it into the filtration.