Two types of concrete posts are encountered in benchmark hunting.
One type is a concrete ["witness post"]. Generally these are 3 inches square and around 2 feet tall. Inscribed letters such as USBM are often seen on them.
Another type is a ["monument"] for a benchmark disk. They are usually 6 inches square and project anywhere from a few inches to two feet above the surface of the ground. The disk is mounted into the top.