I would say a minimum 75g. but bigger is always better. IMO the 20 gallon per fish applies to tanks 75g or smaller (footprint wise not the water volume), with bigger tank you can overstock a bit if you want.
also depends on where you are going to keep it, need to make sure if the floor can support it (it gets very heavy after you fill the tank with water, with weight of the tank, stand and gravel/sand you can use 10lb per gallon as a rough estimate) and if there's any space limitation on where you can put it. for piranha the hight of the tank doesn't help much so the length and width are more important.
if you have only a 4ft space then 120g (4x2x2) would be the biggest you can get as far as standard tank size goes. if you can go for 6ft long then get a 125g or 180g and you can add few more rbps with those size tanks.