Answer
$127.3\ \mathrm{ ft}$
Work Step by Step
The diagonal of the square is the hypotenuse of a right triangle whose legs are sides of the square.
Apply the Pythagorean theorem.
$\begin{aligned}90^{2}+90^{2}&=d^{2}\\2\cdot 90^{2}&=d^{2}\\\\d&=\sqrt{2\cdot 90^{2}}\\d&=90\sqrt{2}\\d&\approx 127.3\ \mathrm{ft}\end{aligned}$