Preferably? 75 gallons or larger. But don't listen to what these tank snobs say. They can live in a 55 gallon for life. They are easily adaptable and live just fine in even smaller tanks. Odd shaped tanks are preferred. If possible. Ex. Hexagon, bow front, etc... pick a FH with a shorter body. They look nicer anyways.
Your kinda in the wrong place asking for tank size. Lol. MFK'ers will tell you the maximum sized tank possible... Any fish is happier in a big tank than smaller tank... Will you stunt a FH? That will be hard. They are like oscars. They will grow big even in a ten gallon, and live pretty long too. I've seen it before. Is it ideal? No. Ideal for a FH is if you build a nice pond and let them live not these monsters... With live food, mates, crazy waterchanges, etc... MOST, but not all MFK'ers are hypocrites. Ask them what they house their fish in... It's nothing compared to what they live in the wild...we keep our fish in pools compared to wild fish... Some have big tanks but look at the biomass. No argument needed... Just keep up with waterchanges for a healthy FH. It's s man made fish. Period. If your willing to do the work to take care of a FH. Get yourself the biggest tank you can afford/fit... And post pics! Good luck!