You can retrieve the current status of a capture window by using the WM_CAP_GET_STATUS message (or the capGetStatus macro). This message retrieves a copy of the CAPSTATUS structure with the current values of its members. The CAPSTATUS structure contains information regarding the dimensions of the image, scroll position, and whether overlay or preview of the image is enabled. Because the information represented in CAPSTATUS is dynamic, your application should refresh the contents of the structure whenever the size or format of the captured video stream might have changed (such as after displaying the video format of the capture driver).
Changing the dimensions of the capture window has no effect on the dimensions of the actual captured video stream. The format dialog box displayed by the video capture device driver controls the dimensions of the captured video stream.