A pair? You won’t find a small male. It’ll be like 5+ inches I’m guessing. They get pretty big. You could get by with one small female for awhile, but I don’t think a pair would be a good idea for a 75g.
Do your research. Everyone has an answer. A regal tang tank size suggestion is 180 gallons. But if you had a 120 or a 150 that were 6 foot long you could also have a regal tang. Make sure they appropriate swimming and living space, and the options are endless.