Bichirs don't seem to take up any space in the tank besides the bottom and they don't seem to mind being crammed in there. All depends on the filtration also.
A sene can live in a 30 gallon for life IMO.
P. palmas palmas ('palmas' bichir)- 13"
(subspecies)P. palmas polli ('marbled bichir)- 14"
(subspecies) P. palmas buettikoferi ('buettikoferi' bichir)- 13"
P. delhezi ('armoured' or 'delhez's' bichir)- 14"
P. senegalus senegalus ('Senegal' or 'Cuiver's' bichir)- 12"
(possible subspecies)P. senegalus meridionalis- 12"+
There is, Also the possibility of a third member of the Polypterus senegalus
group that is found in the Nile River
P. retropinnis ('retropinnis' bichir)- 14"
P .mokelembembe
P. weeksi ('week's' bichir)- 20"
P.teugelsi (found only in the Cross river)-16.75''
E. calabaricus ('Ropefish')- 15"-30" (Closer to 15''-20'' in captivity)
http://www.monsterfishkeepers.com/forums/showthread.php?t=65770