Add custom image size and display on media size selection.

custom.php

<?php

// AB Post Grid image thumb
add_image_size( 'ab-blog-thumb', 337, 224, TRUE );

add_filter( 'image_size_names_choose', 'ab_blog_thumb_size' ); 
function ab_blog_thumb_size( $sizes ) {
    return array_merge( $sizes, array(
        'ab-blog-thumb' => __( 'AB Grid Thumbnail' ),
    ) );
}