Just as a followup, after playing with this plugin, it doesn't do what I was hoping it would do.

While seeing what different image sizes already exist for a particular image is handy, it doesn't actually add the alternative image sizes is use in the theme beyond the ones defined in Media Settings.

The snippet I've been using does exactly that, allowing me to select from any defined image size to insert into a post or page, and that's been invaluable for me over the years (something which WordPress should have added by default years ago, but I won't digress here) :)

I can see where this plugin would be useful while undergoing a cleanup, but unless it eventually includes a way to selectively delete unneeded sizes for selected images, or adds the additional image sizes to the dropdown so they can be selected when adding images to posts, I honestly can't see any other way to use this plugin.

Guess I'll continue using that snippet so I can choose which alternative media size when adding images to posts :)