how do i find the diagonal measure

Diagonal of a rectangle of lenght l and width w :

$${\sqrt{{{\mathtt{l}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{w}}}^{{\mathtt{2}}}}}$$

Note: For a square, l=w so:

$${\sqrt{{{\mathtt{c}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{c}}}^{{\mathtt{2}}}}} = {\sqrt{{\mathtt{2}}{\mathtt{\,\times\,}}{{\mathtt{c}}}^{{\mathtt{2}}}}} = {\sqrt{{{\mathtt{c}}}^{{\mathtt{2}}}}}{\mathtt{\,\times\,}}{\sqrt{{\mathtt{2}}}} = {\mathtt{c}}{\mathtt{\,\times\,}}{\sqrt{{\mathtt{2}}}}$$

http://www.mathopenref.com/rectanglediagonals.html

probably using Pythagoras's theorem.