Most butterflies get over 5 inches, so they aren't what people would usually call small. A yellow long nosed butterfly is typically thought of as one of the easier fish to keep, but they do get kind of big, and I think the recommended minimum tank size for one is about 55 gallons, so you'd be pushing it. I have one in my 90, and he is a pretty active swimmer. So a butterfly recommendation would depend on the amount of swimming room you have, and length of your tank. Here's mine:
He's not as small as he looks there.