Add New Featured Image Sizes To WordPress

By default, every image uploaded via WordPress Media manager will be uploaded to different sizes. However, when displaying them as featured image, the image needs to be re-sized or cropped to get the desired size. This code should go in your functions.php file of your child theme (Yes, I strongly recommend you to use a child theme to make any customization to your theme).

Here is the parameter you need to register a new size of the featured image:
For more details, see the WordPress Codex.

<?php 

// Add new featured image size

add_image_size( $name, $width, $height, $crop ); 

?>

The last parameter, $crop is should be set to TRUE for hard cropping. By default, it returns false.

Here are some examples:

<?php

// Add new featured image sizes

 add_image_size( 'grid-thumbnail', 236, 157, TRUE );
 add_image_size( 'single-post-thumbnail', 236, 236, TRUE );
 add_image_size( 'video-small', 110, 73, TRUE );