This document describes a simple scheme that can be used to write a macro to resize a window using the grow box. The problem with dragging from the grow box is that the Drag command uses window coordinates relative to the top left corner, and the grow box position depends on the window size. So, how do I drag from it? The solution is to bring the window to the bottom right corner with MoveWindow, and then drag from an absolute screen position that's known to be inside the grow box. Here's an example:
MoveWindow bottom right
Drag from 630 470 screen to 10 10
The above macro works on a 640x480 screen (i.e. a 13" or 14" screen) by taking the window to the bottom right corner and resizing it to its minimum allowed size. Since the minimum size is always the same for a given window (although it obviously changes depending on the window kind) you can then resize the window by moving it back to the top left corner and hitting its grow box using absolute screen coordinates again as follows:
MoveWindow top left
Drag from XXX YYY screen to 100 100 relative
(where XXX and YYY should be found by using the WhereIsClick extension on the target window after it has been moved to the top left corner as done by the macro).