<?php
/** register additional image sizes https://developer.wordpress.org/reference/functions/add_image_size/
** you may need to regenerate your thumbnails with a plugin such as https://wordpress.org/plugins/regenerate-thumbnails/
** after adding
**/
add_image_size( 'my_custom_image_size', 300, 200, true ); // 300 pixels wide by 200 pixels tall, hard crop mode