The WinGetMaxPosition function fills an SWP structure with the maximized-window size and position.

#define INCL_WINFRAMEMGR /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HWND    hwnd;      /*  Frame-window handle. */
PSWP    pswp;      /*  Set window position structure. */
BOOL    fSuccess;  /*  Success indicator. */

fSuccess = WinGetMaxPosition(hwnd, pswp);