This example calls WinQueryWindowRect to get the dimensions of the window, and then calls WinSetRect to downsize it.

#define INCL_WINRECTANGLES
#include <OS2.H>
HAB hab;
RECTL rcl;
HWND hwnd;

 WinQueryWindowRect(hwnd, &rcl);       /* get window dimensions */
 WinSetRect(hab,&rcl,
            rcl.xLeft - 10,
            rcl.yBottom -10,
            rcl.xRight - 10,
            rcl.yTop - 10);