I find these fish to be really cool--they're true oddballs, especially with the 'wings' that are responsible for their name. Also, you know a fish that ugly must have a good personality!
They only grow to about 5 inches tops and 4 is much more likely in aquaria. It isn't so much a matter of tank size as it is of surface area--they will never leave the surface. A tank with length and width of 24''x12'' respectively should suffice (a standard 20 gallon is 24''x12''x16'') so long as the butterfly fish is the only surface dwelling fish (they aren't generally aggressive but they are territorial.)
In terms of temperature, standard tropical range is fine and in my experience they aren't too sensitive regarding water conditions. Some guides list that they like soft, acidic water but mine is thriving in my tank with a ph of 7.6.
It is imperative also, as with most insect-eating fish, to have a solid cover on the tank with no holes big enough for the fish to fit through. Not only are they jumpers, but they can glide--stories of peoples butterfly fish going missing and being found 6-12' feet away are not uncommon.
In terms of keeping them happy, they thrive in still water so make sure there's some part of the surface that isn't being agitated by a filter, air stone, spray bar, etc. They also appreciate floating plants as they can be skittish (though I find plants to be more trouble than they're worth so I go without them and my fish is fine.) When feeding, make sure that you provide food that won't sink (I use freeze dried bloodworms) as they won't chase food below the surface, even if it means starving.
They also don't swim too fast so their hanging fins are easy targets for any fin-nipping medium dwelling fish like gourami's and angelfish.
Sorry, I know this is way more info than you asked for, but it's almost never that anyone asks about a fish that I have actually successfully kept
