resize related posts in mobile

You can customize the look of every element on your site thanks to CSS. You can add that CSS to your theme stylesheet, or to your custom CSS editor available under Appearance > Edit CSS in your dashboard.

To change the size of an element only on mobile, you’ll need to use CSS Media Queries:
https://css-tricks.com/snippets/css/media-queries-for-standard-devices/

If you’re not sure how to target just the Related Posts, you can follow this tutorial to learn how to target specific elements thanks to CSS:
https://dailypost.wordpress.com/2013/07/25/css-selectors/