*buzzer sounds*
Nice try contestant, but the correct answer is a volitans will definitely outgrow a 55 gal setup. In fact, our volitans has outgrown 3 tanx since we got her as a tiny 1.5" juvie. It now resides in the spacious setup behind the curtain where the lovely seahorsedreams is standing...
OK...so it's been a long day and I'm punchy. However, in all seriousness, a full-grown volitans really should be in a setup that's at LEAST 18" wide, and is properly aquascaped to allow sufficient "turnaround" room. In fact, a 24" wide setup like a 48" x 24" x 20" 100 gal or 48" x 24" x 24" 120 gal both make super nice setups for large-bodied lionfish.
IME, when lions have sufficient room to swim around, they do so, and they are much less "boring" as some folks put it. They really do become different fish, and nothing is as impressive as a full-grown volitans cruising regally about.
HTH