Tell them to double click the title bar of the window. This will maximize it for most windows, similar behavior to what they should be familiar with in Windows on that front at least. Sometimes this results in a "smart" maximization, like if it's a PDF in Preview it'll take all the vertical space it can get, but won't widen if it's in a single page view beyond what's needed. You can also hover over the edges (left/right, or top/bottom) until you get the double arrow cursor and double click to extend in horizontally or vertically, again just like Windows offers there.