The size box on a window allows you to change the size of the window.
The size box must be clicked on for it to change the size of a window. Move the pointer over the size box and click -- and hold down the click -- and start dragging to change the dimensions of a window. You can drag horizontally, vertically or diagonally, and you can drag up, down, to the left and to the right, as well as in all compass directions.
In earlier versions of the Macintosh system software, an outline of the window appeared. Beginning with Mac OS X, however, dragging on the size box of a window can also make the window resize in real-time, although some Classic programmes slightly rewritten in Carbon still do outlines.