You might be able to keep a Protopterus amphibius LF in a 55 for life provided the tank has top-of-the-line filtration on it. However, finding a P. amphibius can be a daunting task. They don't ship well, are difficult (at best) to keep healthy, and can cost as much as a new ]V[onster aquarium setup.
A 55 will house most LFs for their 1st year of life. However, they'll all require tanks double the size of a 55 (and up) in quicker time than one might think.