Batteries are a power source.
They can be considered a source of a VOLTAGE (1.6V approx) with a resistance in series. The resistance in series is due to the battery not being perfect. The effect is that the higher current you draw, the lower the voltage. (Also the voltage decreases with state of charge).
Generally speaking, larger batteries are "better" and have a lower equivalent series resistance. They also have a larger capacity.
The former means that they will supply a greater current for a given voltage drop.
The latter means they can supply a given current for longer.
The capacity of batteries is given in mAhr, which is a combination of current and time, i.e. 1200mAhr means a battery could supply approx 100mA for 12 hours.
Manufacturers of batteries will know the mAhr rating of their batteries, and if you are lucky, they may publish it.