By default the Related Products option uses Contain to display images 

Pro Blogger: OS 2.0 Theme - The difference between Image Display: Contain and Cover

This ensures the full product is in visibility at all times but for styling purposes it may be preferable to use a cover option and fill the entire window with the product image.

To apply this styling option - simply add the following custom class to within the Theme customisation option


As per the example below and then click 'Save'

This will change the image display for that instance of Related Products slider