depends, fu manchu ive read 30 gallon, i keep mine in a 75.
for anyother, ive read 55 gallons, but id say 75 or higher they get pretty large especially volitans.
If you want a volitan and want to grow it out to max size i would do something over 100 gallons. They get huge like a football. Wont be very comfortable in anything less than a 100.
I've seen several tanks devoted to lionfish, and they are something else. Tank size depends on what kind of lionfish you are interested in. Volitans will obviously need a bigger tank as an adult than a radiata or fuzzy zebra.
Thouht I might update you guys with some research. From what I've read, Even though lions are notoriously lazy and inactive during the day, they do need alot of horizontal swimming room. The minimum tank size I keep seeing is 55 gal with 110 gal at least being ideal for the larger ones. So i guess the lion tank will have to wait for a while. I'm working on a reef and don't have room for another large tank.