From the owner's manual... These are typically group size 27 batteries.
The DC emergency pump system requires a good quality, 12 volt battery to obtain maximum pumping time during a power outage. A deepcycle, 12 volt, 105 amp-hour marine battery or larger is recommended and will provide approximately 7½ hours of continuous pumping time in a sump pump installation with 8' of head pressure. In most installations the pump runs intermittently and the battery life is extended accordingly. Batteries with top terminals are recommended for ease of installation. These batteries contain acid and proper precaution must be taken when handling. Maximum battery size 12½" Length x 7" Width x 9½" Height