Turquoise severums will get to the same size as all severums, so look for a 4ft tank for the pair. The males can reach 10" and females 8". They can be grown on in a 25 gal tank then you must upgrade to the 4ft. Do you want to know anything else?
I'd get the recommended tank size for their adult size, as they grow quickly Just as quick as oscars if you ask me (I've kept both).
I kept mine in a 180g - but that was with several other inhabitants. They say about 60g min for a pair, but preferably nearer 90g, but I'd think a 135g should do it for 4, then you could have some dithers. I wouldnt go much smaller though, some sevs grow into monsters - 8 and 10" is the average, but they can get up to 12".
The bigger the tank the better - especially to start off with. Ours grew quite fast initially in a 300l/60g, but when we moved them into the 180g they had a huge growth spurt - they went from 1.5" to 5" in about 3 months. Sold them now but they were still growing fast when they left!