The problem with 55 gallon aquariums is that they're so narrow. They may be 4 Feet long, but they're only 1 foot wide and larger animals would feel cramped. I've seen them used pretty successfully though by people who would stand them on their end and house some climbing geckos. I've got a friend who likes to use those for pairs of tokay geckos.
You know i think it is a 60 gallon so it has a little extra depth. I will measure when i get home.