Make medium_large images available to insert into post

since I’m using a size already generated by WP, I just needed to add:

add_filter( 'image_size_names_choose', 'fresh_custom_sizes' );
function fresh_custom_sizes( $sizes ) {
   return array_merge( $sizes, array(
      'medium_large' => __( 'Medium Large' ),
   ) );
}