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
img-cover
As per the example below and then click 'Save'
This will change the image display for that instance of Related Products slider