Scale? Look at the shelf near the top center of the rock (about 3/4 of the way up). That small dark blue spot on the center of the shelf is my hat.
Had this boulder tumbled onto a passing canal boat, it would have kinda ruined the captain's day, doncha think?
As for Ultrarunning, I've never been menaced by a falling rock. Rocks certain can and do fall, but getting struck is probably about as likely as being struck by lightning. My rock problems consist mostly of trying to run on loose rocks. Usually I am quite surefooted on fixed rocks and sometimes can scamper over them like a mountain goat.