its diameter is 16 inches,
So, according to the Pythagorean Theorem, for some right triangle with legs of length a, b, and hypotenuse of length c,
then a² + b² = c²
c = 16 inches, and a = b.
a² + a² = 16²
2a² = 256
a² = 128
√a² = √128
a = √128
a = √(64•2)
a = 8√2
So, the size is an 8√2 by 8√2 square.