בסכת תשבו שבעת ימים
father a”h told me, he heard R.M.B. Weissmandel zt"l giving a
Shiur on Hl. Succah and he said
"The minimum size for a kosher סוכה is 7x 7 טפחים. The diagonal of 7x7 = 9.89949494 טפחים
R.M.B. said that the גמרא עירובין מ"ח ע"א a says that the average height of a person is 3אמות (18 טפחים)*
A person who is 18 טפחים his ראשו ורובו would be 9.1 טפחים.
For an average sized person to fit his ראשו ורובו laying down on the diagonal of the Succah. we would need the סוכה to be at least 9.1 טפחים **plus a few more millimeters diagonally
Hence, the reason for the minimum size of a סוכה is 7X7 טפחים