As big as you can get. I'd say anything over 55g. But a 6' tank will reduce any aggression. IME they are less shy in a 6' tank too. But be careful with breeding pairs as they get really aggressive.
I have a 5" female salvini in a 150g with some thorichthys, h. Deppii, c. Robertsoni, c. Rostratus, HRP, rainbow, and 2 spot astyanax tetras. I have zero aggression from the salvini. In fact she gets chased by firemouths half her size. I have never had a really aggressive salvini, but wondering if that's because of tank size.
IME they stress easily and are very prone to bloat or HITH. Make sure a varried diet is offered. While they are primarily preditory cichlids, algae or plant matter has been found in the stomachs of wild fish.
Here is my peaceful girl.