Simply put - the bigger the better. There is no limit to the amount of space they need. An enclosure cannot be too big, but it can be too small. Big big big
My 3, 16month old leopards are in a 17'x8' pen when outside during the day.
My 2 new hatchlings are in a 5'x3' pen when outside for an hour or 2 each nice day.
If you can provide a nice large area, do it. Everyone always says the same thing after making their pens, "I wish I would have made it larger"
If you want exact measurements my 6 inch leo has a 7ft x 3ft outdoor cage with a roof and blocked off sides. But when I have the time to sit out with him he gets the run of mine and my next door neighbours gardens.