Make gallery images size customizable in the editor

[GregLone]

Hello.

Now the default parameters for the galleries can be customized (see #28693), it's not really difficult to allow the images size to be overridden.
The only thing we need to do then is to add our custom size with the "available ones":

add_filter( 'image_size_names_choose', 'custom_image_size_names_choose' );
function custom_image_size_names_choose( $sizes = array() ) {
	$sizes['gallery-thumb'] = __( 'Gallery thumb' );
	return $sizes;
}

The only thing we should be aware of is that the "size" parameter is added to the shortcode (imho it shouldn't, because this custom size may not be good for any theme we install afterwards).

[michalzuber]

Rewrite to applicable via grunt patch

[GregLone]

Sergey, thanks for the description modification, I saw my mistake too late and couldn't edit it.

[wonderboymusic]

Duplicate of #18143.

the default value is always set for the shortcode now, and can be overridden in the Gallery Settings UI: since [29779].