The MCIWNDM_NOTIFYSIZE message notifies the parent window of an application that the window size has changed.
MCIWNDM_NOTIFYSIZE
wParam = (WPARAM) (HWND) hwnd;
lParam = 0;
You can enable notification of changes in the size of an MCIWnd window by specifying the MCIWNDF_NOTIFYSIZE window style.
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Unsupported.
Header: Declared in vfw.h.
MCIWnd Window Class Overview, MCIWnd Messages