I was advised a minimum of 3ft, ideally 4ft long, when I set up a pulchra tank some years ago. If they breed, you will want room for the fry, so the bigger the better, as always, with and fish. I personally don't think it's best to go by how many litres/gallons, as height isn't a massive factor for snake heads. Minimum width of the tank atleast the max length of the fish and 4 times the max length of the fish for length of the tank is what I aim for.
