If you want to keep it for life, I'd say a minimum of 8x4x3, but tbh that's still alittle small, especially as they will hit 36" given a good diet and a few years.
My 3'-er is in a 13'x13'x4.5', 4500 gal and it looks ~ adequate. I'd not even call it great but just adequate. My prior 3'-er was in a 40'x6'x2.5'. It was too shallow, too narrow IMHO. It didn't swim nearly as much as my current one does.
Ripsaw will reach 3' in less than 4-5 years. Gentle giants for sure but they need space. They are poorly flexible with an enormous girth relative to other fish.