It seems that the parameter "metadata" will just add the metadata to the img tag but it will not select a specific thumbnail.
I wonder if it would be a good idea to add another parameter like "thumbnail" where the user can either enter a thumbnail name like "small" or a specific number like "600, 350" instead of just using "featured-slide" in the_post_thumbnail(). It will prevent problems when the aspect ratio is different on the custom size, from what is defined in featured-slide.
Would it be a good idea to modify the plugin and ask if this could be included in future updates?
How do I do that? Do I have to contact the author of the plugin?