adjusting height according to content length?

You could give the whole timeline more height using CSS. In fact you can take a lot of control over the presentation — this tutorial gives some guidance: https://tutorialzine.com/2012/04/timeline-portfolio/

There’s already a lot of dynamic configuration for the size and position of different elements, and it’s very difficult in HTML to know how much space text takes up in order to size something to guarantee that the text fits. I think in this case you’re better off considering editing your text. Since the primary use case for TimelineJS is story telling, you might be doing your audience a favor by trimming your text to the essentials anyway.