There is no real magic number. It's all a preference, but the most important thing is. It Must be big enought to hold all the drain water when the main pump turns off... Me, I like a big skimmer to skim out the waste, because i like to feed my fish n corals very heavily. So i got a sump that had a big enough area for a bigger skimmer. you would also want a refugium area so you can raise pods. Also, be aware that you would want to store your equipments. Reactors, pumps, ATO boxes, Dosers, Calcium reactors, Aqua controllers... etc etc. so I would give myself a godd portion under my tank for all that. UNLESS you have a room behind the tank and space isn't an issue.. then, the bigger the better. But if you want to hide all the equipment. leave yourself some room.
Example: You have an area under your sump that's 70" x 17". I would make my sump 48" x 16". That would leave me about 22" for all my other equipment. Or shelves to store my chemicals, fish food, etc etc.
In the end. It's your preference, just as long as it can hold all the water that gets drained down. That's what a sump is primarily used for anyways. a place to store your equipment away from sight. Skimmers, reactors, filters.
I try to have at least 30% of the water volume in the sump so that I can basically empty it for water changes. I do run a refugium though. More water typically = more stability. What type of corals do you plan on keeping?