VideoInputFrameGrabber use the default format

I'm trying to capture images from a webcam, I've tried using OpenCVFrameGrabber and FFmpegFrameGrabber, they give me errors. Now I'm using VideoInputFrameGrabber, I found out that I have to set the image width and height, otherwise it says ERROR: SampleCB() - buffer sizes do not match. It has already printed the default format in the console:

SETUP: Setting up device 1
SETUP: MJPEG Camera
SETUP: Default Format is set to 2048 by 1536 
SETUP: trying requested format RGB24 @ 640 by 480
SETUP: Capture callback set
SETUP: Device is setup and ready to capture.

2048x1536 is my webcam's size. If it sets the format to 640x480, I get a ERROR: SampleCB() - buffer sizes do not match. Could anyone tell me a way that can make VideoInputFrameGrabber set the correct image width and height automatically?