This call returns the length of the window text, excluding any null termination character.

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

HWND    hwnd;     /*  Window handle. */
LONG    lRetLen;  /*  Length of the window text. */

lRetLen = WinQueryWindowTextLength(hwnd);