Auto-size is an easy and smart way to resize your text without it affecting other visual elements in your experiment.
Text elements using Auto Size automatically change their size to always fit the text box.
It's useful because you can put in text assets of varying length, and it'll automatically look good without having to fuss with it.
You can enable Auto Size by clicking on the text you want to work with, and then toggling on the Auto Size button in the right-hand elements panel:
When you turn the Auto Size on, the font size selector will change to percentage-based.
That percentage represents the percentage of the area in the text box your copy will occupy.
Here's. quick animation showing Auto Size in action: