Assume you have "all kinds" of wind, and can choose the day where it is okay, meaning not too much for mountainboard, and sufficient on the water.
Weight is not that important, if you can choose the wind days yourself, but size is.
A 4 m2 would be the best choice on asphalt or mountainboard, and a 5 m2 for wingfoil.
So 4½ to 5 m2 would be the best size if you are average weight and lots of wind.
5 to 5½ m2 if you are heavier and have less wind quite often.
If you only want one for starters.
No matter what, something around the 5 m2 is the best to start with