Severums rarely exceed 9 inches, but they are a tall-bodied, thick fish. A 4 foot tank is plenty for a pair. A standard 75 or 90 gallon tank would be perfect.
I would say that a 75 gallon would be a good place to start. If you could get a 100 to 125 gallon that would be ideal IMO. The more swimming room they have the better I think. Good luck.
I kept an almost full-size breeding pair in a standard 55 for over a year, no problem. A little more turn around room would be ideal though, which is why a 75 (or even a 65) would fit the bill nicely. A 6 foot tank, while nice, IMO is wasted on just 2 of these fish. However, with the addition of a large pleco, a school of 5 or 6 silver dollars, maybe an anostomus, etc....
To my knowledge all severums are basically the same except for the color pattern. There is the green, the gold (albino) and turquoise. I think there is also another severum-like cichlid that is very rare in pet stores but it is classified in a different genus (for whatever reason). We'll probably never see it in person.