The worst thing you can do is cover a tree's roots with ANYTHING! Soil, rocks, stones, more than 1 inch of mulch... yeah... anything. It would be like putting a pillow over your face. You would suffocate. So do they. Tree roots NEED AIR!
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!