There are many big trees at Calaveras Big Trees State Park but none rises so majestically as the Agassiz Tree. The base of the Agassiz, a giant sequoia at Calaveras Big Trees State Park, believed to be 2,000 years old, is 250 feet tall, 25 feet in diameter (measured 6 feet off the ground) and has a circumference of 97 feet on Monday, Sept.