I'm looking for a way to have a slideshow that keeps its proportions as the browser window is resized, without being full width. Something like this: http://cigue.net/en/project/the-book/
Is this possible? if not, are there any good close alternatives/3rd party widgets that would work?
The responsive slideshow widget as per your description is our top priority for Muse development cycle and might come in new future.
However, as of now, you can choose any of third party widgets available here responsive slideshow – Adobe Muse Widget Directory.
Hope this helps.