Adding new size for Media screen in functions.php not working

Change your function to this.

function wpshout_custom_sizes($sizes) {
  $sizes['medium-width'] = __( 'Medium Width' );
  $sizes['medium-height'] = __( 'Medium Height');
  $sizes['medium-something'] = __( 'Medium Something');
  return $sizes;
}
add_filter( 'image_size_names_choose', 'wpshout_custom_sizes' );