Are you looking for a way to set the default thumbnail size? While there’s probably a plugin for this, we have created a quick code snippet that you can use to set post thumbnail size in WordPress.
All you have to do is add this code to your theme’s functions.php file or in a site-specific plugin:
set_post_thumbnail_size( 100, 100 );
Note: If this is your first time adding code snippets in WordPress, then please refer to our guide on how to properly add code snippets in WordPress, so you don’t accidentally break your site.