Dwarf lionfish species actually do fine in reef setups, and considering that they only need food 3x a week, they really don't put as much load on your water as reef fishes that require lots of food in multiple feedings. Also, a 55 gal will be a fine size for a dwarf lion.
As for the terminal size of the fish, if you get a full-grown adult, it may come in at 7" TL, but IME, most of the specimens raised in captivity will be more of a 5" - 6" fish TL.
For a first dwarf lionfish, I'd highly recommend a fuzzy dwarf (
Dendrochirus brachypterus) over a
D. zebra (you didn't specify which fish you saw), as they're more robust, and are much easier to feed in most cases. Fuzzies also come in different color morphs (brown, red, yellow, or any combo thereof), and tend to be more "personable" than the zebras.
The only real issue with lionfish and reef setups is that they can limit your tankmates to fishes that they can't swallow. Lions will also eat ornamental shrimp and crabs, altho a CUC of snails and hermits (I prefer the scarlet reef hermits) will be fine.
Getting the fish eating and weaned may take some patience, or it may happen quickly, but as I mentioned, most fuzzies are pretty easily weaned. What I'd do at the LFS is check the fish to see if it will take live guppies or ghost shrimp. If the fish does that, then it will eat for you, and getting it weaned will be your next job.
Here's a good primer on lionfish care:
Lions, and Turkeys, and Devils, Oh My! Lionfish in the Home Aquarium
Here's an article regarding weaning predatory fish...I highly recommend making the "stealth stick":
Tools of the Trade: Equipment & Techniques to Convert your Fish onto Frozen
Give a shout with any questions you may have.