Only set height

No, I think is better to write your own css code to display images in menu. Plugin is good only for simple usage.

But it’s possible to change the image size, right? Wouldn’t it be quite simple to have something like height: 50px and width 100%?

I’m not experienced enough to write that kind of css on my own unfortunately. Thanks though.

You can run add_image_size('menu-h48', 9999, 48); with really big width and select this image size in menu item settings.

I think I did something wrong, so I removed the code. Where should I add it?

Add it to functions.php in child theme:

add_action('init', 'add_custom_image_sizes');

function add_custom_image_sizes(){
  add_image_size('menu-h48', 9999, 48);
}

Great, thanks so much!