Said to be destroyed in 1869 and I can't find much about it but see the sheep for scale!
http://www.fromoldbooks.org/OldEngland/pages/0041-Constantine-Tolman,-Cornwall/
“It is one vast egg-like stone thirty-three feet in length, eighteen feet in width, and fourteen feet and a half in thickness, placed on the points of two natural rocks, so that a man may creep under it.”

Was it man-made and was it that size?