hi sushi,
glad to hear of your interest (or at least your fiances interest

) in lungfishes. as previously stated, all species (there are 6 total) will have the potential to get larger than 12"-18". the smallest species is actually the rarest, Protopterus amphibius, and only one has been brought into the US in recent history (moderator Oddball and myself had the privilege of keeping that fish for a time).
you can see positively ID'd pics of all the lungfish species in the lungfishes section of
www.primitivefish.com, along with all their wild maximum sizes.
if you would like a lungfish that will get along with other fishes, i would highly suggest the South American lungfish (SAL), they can grow to be quite large, but are slender and don't need a ton of turning room (a 24" wide tank would do, but a 36" wide tank is preferable for a large adult...it would of course take a year or two for them to reach the size where this is needed). they do, however, get along well with other tankmates which they cannot swallow (this is a generalization, and the individual personality of the fish must be taken into account as well).
another peaceful lungfish is the Australian lungfish (AUL), but these are quite rare and very expensive (around $800 minimum for one fish). they are generally more peaceful than the SAL, but again, their rarity and price make them a bit less practical.
the other four species are African lungfishes, all of which generally do not get along with any tankmates, and should be kept on their own. the logical choice here is the Protopterus annectens (West African lungfish or WAL). these are commonly available in pet stores (at least in terms of lungfishes), and develop a nice pattern and could live for life in a standard 180g tank (6 x 2 x 2). they can be moved up in a stepwise fashion, so your 75g would house a smaller individual for a couple years before an upgrade was needed. again, they should be kept on their own.
hopefully that is enough selection info to get you started if you both decide that you do indeed want to get a lungfish. they are very hardy and nearly indestructible fishes in most regards.
since you mentioned teeth...they don't have teeth in the way we think of them, but have ridged tooth plates which are used for crushing/mashing up food. these ridges have edges which do make them sharp in a sense, however, and if you watch them take down a whole fish or shrimp, you will see what kind of damage they can do...and they strike very fast. a large lungfish could easily crunch off one's finger if they get careless around the fish in the tank!
hope that helps get you started, and let us know if you need more info (provided you decide to go forward with the lungfish project) --
--solomon