I ended up doing this:
add_filter( 'image_size_names_choose', 'jss_custom_image_sizes' );
function jss_custom_image_sizes( $sizes ){
$custom_sizes = array(
'customsize1' => 'Post Rectangle Image',
'customsize2' => 'Post Square Image',
);
return array_merge( $sizes, $custom_sizes );
}