What determines the size battery needed (or used) in a device such as a flashlight?

How long might it be expected to last, like on a hunting trip, how compact is it /where will it be stored for potential use, how well will it fit the hand, what it’s expected use is/might be.

It’s an engineering trade-off. Physical size, power requirements, manufacturing costs and marketability key into those decisions. When is the last time you saw D cell batteries used?

How effective you want the flashlight to be as a club.

@Zaku Maglite ML300LX LED 3-Cell D Flashlight

I want to give @Zaku 2 GAs, one for clever, the other for a single sentence post!