Hi Pierro,
You can really do it in simple HTML and CSS... Really.
Trust me : http://codepen.io/anon/pen/pjLGVr
Maybe I am wrong on your problem, but you only have to set your slide-box with a fixed height (like 100%) and you put scrollable div in it for your content, no matter the length of it... that's it ! No javascript, no jquery.
Best