The battery has a diameter of 13 mm and is 25 mm tall; roughly half the size of a AA battery. It is denoted simply as 28A by Duracell, or 4LR44 if of alkaline magnesium chemistry, or 4SR44 if silver oxide.
The charge capacity of the alkaline battery is stated differently by different makers: Duracell's 28A alkaline cell is said to have a capacity of only 105 mAhr; Camelion claims a capacity of 150 mAhr for their 4LR44, and 145 mAhr for the 4SR44. The silver oxide battery has a slightly higher voltage of 6.2V.
The battery is functionally equivalent to four 1.5-volt LR44 or SR44 cells, though four of these are smaller than the single 6-volt cell. Depending on the design of the battery compartment it may require padding if they are to be substituted.