N. Notopterus averages 12" in aquariums. They are the most belligerent of the Asian Species. I think they would be fine in a 29g, personally, but 50g would be best.
asian knife fishes are quite easy to keep because they can tolerate bad water conditions but they are aggressive to smaller tankmates. mine killed its tankmate, a red pacu. but it got along well with my bichir.
currently lmfao @ bikini bottom comment, will brb to post comment.
like benzjamin said you could get away with a 29, (knife fish dont always swim the long way like pike, gar, oscars) so a 12" knifefish can fit where an 8 inch other fish would be snug. but a 55+ would be optimal