To crop, you simple specify how many pixels will be cut from each side (left, top, right, bottom) of the original frame size.
Once cropped, the resulting frame size will be resized and centered to fit output size (if necessary), as usual.
Example: the source file size is 704×576, and we want to crop 10 pixels from each size. Just set 10 for top, left, right and bottom. So, after cropping the frame size is 684×556. Assume we set the result size to 640×480. Then, the frame will be resized to 590×480 and 25-pixels pads will be added to the left and to the right of the frame.