Resize by Pixels - Check this radio button to resize all batch images to a certain width or height.
If Keep Aspect Ratio is checked, only one dimension can be specified.
If Auto Height is checked, images will be resized to the width provided, and the height will be automatically set to a value that preserves the aspect ratio.
If Auto Width is checked, images will be resized to the height provided, and the width will be automatically set to a value that preserves the aspect ratio.