If you look carefully, that diagonal would be the Hypotenuse and the arms of the Square (which is equal in magnitude) would be the shorter sides.
Then, According to Pythagoras Theorem,
a² + b² = c²
2a² = 8²
a² = 64/2
a = √32
Now, Area = Side²
So, A = (√32)²
A = 32 in²
In short, Your Answer would be; 32 in²
Hope this helps!