The size of Water Lettuce depends upon the environmental conditions in the aquarium (or pond). I had this plant in my outdoor pond in summer, and it grew to about the size of a dinner plate. But in my aquariums, it remained small in some and slightly larger in others.
The plant species is Pistia stratiotes. In good conditions, this plant will grow large, up to 50cm / 20 inches across, though usually smaller. The "dwarf" form, which is not a separate variety, subspecies or distinct species, is believed by most sources to be due to less than ideal conditions; observations have noted that the "dwarf" plants develop into normal large plants when conditions improve. Light is a major factor, which is one reason my aquarium plants grew so large in the outdoor pond under summer sun.
It may do very well in your 5.5 gallon tank. One nice thing about this plant is that it rapidly produces daughter plants, and if it does get too large, take the parent plant out and leave the daughter plants, which will grow to a similar size, again depending upon light and nutrients. It will probably be better with a comprehensive liquid fertilizer. Fast-growers like all floating plants are need a good supply of nutrients.