The DxO Optics Pro manual has the following to say:
If you enable the Resize Image option, you must select from the following parameters:
- The maximum size of the output image (whether height or width) in pixels, centimeters, or inches. A single dimension is sufficient for the program to maintain the aspect ratio (proportions) of the image.
If you want to keep the aspect ratio of your exported images, make the adjustment here. If you want to change the aspect ratio instead, you must crop the images to taste before exporting. With AFAIK the Optics Pro, you can not automatically do this for all images in a specific folder.