Add custom image sizes to WordPress Media Uploader choices

functions.php

<?php

/** Add Custom images sizes to Media Uploader */
add_filter( 'image_size_names_choose', 'your_image_size_names_choose' );

function your_image_size_names_choose( $sizes ) {

    $sizes = get_intermediate_image_sizes();
    $add_sizes = array();

    foreach ( $sizes as $size )
        $add_sizes[$size] = $size;

    return array_merge( $add_sizes, $sizes );
}