Daf Hashavua Eruvin 48a
My father a”h told me, he heard a Shiur from R.M.B. Weissmandel zt"l on Hl. Succah.
The minimum size for a kosher Succah is 7x 7 Tefachim. The diagonal of 7x7 = 9.89949494 tefachim
R.M.B. said that the Gemoro Eruvin 48a says that the average height of a person
is 3 Amos (18 tefachim)*
A person who is 18 tefachim his Rosho Vrubo would be 9.1 tefachim.
For him to fit in his Rosho V’rubo on the diagonal, the Succah’s diagonal needs to
be larger than 9.1 tefachim because the shape of a human head doesn’t end at a 90’
R.M.B. came up with the exact measurement (The rest, I don’t know)
*According to one pshat in Tosfos it doesn’t include his head.