Here, p.6, an elementary example of design (size) optimization is given.
It's of a tray that has given volume $V$ and height $H$. The design problem is to select length, $l$, and width, $w$, of the tray.
So,
given $lwh=V$ and $h=H$,
A "workable design" is $$lw=\frac{V}{H}$$
What is the meaning of $lw$? It's an area of a rectangle, but how does this define the optimal design?
