Here are the details also revealed about them:
AT-M6 stands for All Terrain Megacaliber Six. Named so for the Megacaliber Six cannon mounted on its back. While the traditional AT-AT walks like an elephant, the AT-M6 is said to have a more ape like gait.
The First Order Dreadnought is equipped with two orbital auto cannons which are used for large scale bombardment. It’s measured at 7,669.72 meters long.