You can know what another living Being's life is like because you can imagine or intuit looking out through their eyes, feeling their roots or wings, or hearing their inner song. It's a non-linear process--you get to let go of being human for a few minutes and imagine being a tree...or some other Nature Being.
You can measure a big tree by simply standing around it and holding out your arms. We call it "Ring Around Tree" and it's a lot of fun! Get someone to take a photo of your family around a big tree...just like this!