Your 55 gal is probably around 36'' long and 18'' deep, correct? The bulb you linked is a T8, so you'll want a T8 fixture. I'd go with at least the 24'' bulb and fixture. If you are mounting the bulb on top of the tank (through mesh) then I suggest cutting the mesh in front of the bulb to allow all of the UVB through tot he basking area. Otherwise, I suggest getting a T5 High Output bulb/fixture around the same length as that will have enough output at the basking site (6-8'' away) even with the screen.