You also need to add it to image size chooser.You can do so using the filter image_size_names_choose.
add_filter( 'image_size_names_choose', 'wpse_228675_custom_size' );
function wpse_228675_custom_size( $sizes ) {
return array_merge( $sizes, array(
'image_horizontal' => __('Custom image size'),
) );
}