Define multiple Gutenberg editor widths

Question

You may need to use the same approach that conditional editor stylesheets require now (for TinyMCE).

function my_theme_add_editor_styles() {
    global $post;
    $post_type = get_post_type( $post->ID );
    $editor_style="editor-style-" . $post_type . '.css';
    add_editor_style( $editor_style );
}
add_action( 'pre_get_posts', 'my_theme_add_editor_styles' );

This snippet is from a Hongkiat article describing the process.

Note: You would then have a different editor stylesheet in your theme directory for each post type:

editor-style-POST_TYPE.css

Using sass/less would make this a little more manageable since you could compile these stylesheets from partials and just add the class that changes the Gutenberg editor width.

Not ideal, but this approach could get the job done.