The WinStoreWindowPos function will save the current size and position of the window specified by hwnd.

#define INCL_WINWORKPLACE
#include <os2.h>

PSZ     pAppName;  /*  Pointer to application name. */
PSZ     pKeyName;  /*  Pointer to key name. */
HWND    hwnd;      /*  Window handle for the window to be stored. */
BOOL    rc;        /*  Success indicator. */

rc = WinStoreWindowPos(pAppName, pKeyName,
       hwnd);