70gal min
I would HIGHLY advise setting up a top feed with 1/2inch tubing and 3/16inch feeder lines. You can use a 70gal and never need to top it up with water until you change the res , usally about 7-9 days. This really helped me break my feeding up into 8weeks, I just changed the res every sunday
if your doing a
Flood and drain, a 100gal with allow the water to go much higher up the medium, but I did it for a year with a 70gal but you need to top it up every 2-3 days so the pumps dont run dry which is annoying to bring the hose in the room ,mixing a little nutes up , then PH'n it. Do top feed its excellent and you only have to adjust the PH every two days and leave the hose for once a week