A slider can fit to the content. However, the slider will be as tall as the tallest slide it contains. Slides and Mask expand naturally, but you need to alter the Slider element for this to work.... and know about a little trick: sometimes in Webflow, a default value (a grayed one) doesn't really work, you have to type it in.
So select the Slider element, give it a class, and in the Height field, where Auto is written in gray and is supposed to be the default, type "auto" in and Enter.
(outch this gif is funky, have a look to the mp4: http://vincent.polenordstudio.fr/snap/ipp5l.mp4 )