Register Custom Image Size (Editor-Selectable)

<?php
if ( ! function_exists( 'scseed_register_card_image_size' ) ) {
    function scseed_register_card_image_size() {
        add_image_size( 'scseed-card', 600, 400, true );
    }
    add_action( 'after_setup_theme', 'scseed_register_card_image_size' );
}

if ( ! function_exists( 'scseed_add_card_size_to_editor' ) ) {
    function scseed_add_card_size_to_editor( $sizes ) {
        $sizes['scseed-card'] = __( 'Card (600x400)', 'scseed' );
        return $sizes;
    }
    add_filter( 'image_size_names_choose', 'scseed_add_card_size_to_editor' );
}