They are fish with much opinions surrounding them.
Some websites say 6 inches as maximum size other say in excess of a foot.
I would say you could go a pair in a 55 gallon, but choose what you keep with them wisely. As they get larger so does their mouth and the other rule of thumb is if it fits in a mouth its lunch.
So kissing gourami will grow larger than most fish you will generally keep in a 55.