My LFS used to have one in a 75 - it was the manager's pet. You might be able to get away with one in something smaller than a 75, but there really aren't any good standard tanks between a 55 and a 75, IMO. A 55 wouldn't be too great either, just due to how thin and short they are. Lionfish will perch and flare, and they really use all that extra space, even though they are sedentary fish compared to most marine fish.
If you get a pair, definitely 75 gallon or more. No way I would put even one in a tank smaller than 4 feet long. They may only get 7 inches max but their spines protrude everywhere and as mentioned they will flare their spines which adds significantly to the amount of space they need.