Originally posted by Hrolf Hakonson
In modern armies, 81-82mm (NATO-Warsaw Pact) mortars ARE the standard. In period, though, as AdmK says, there's a wider range (in fact, in some armies, a 40-60mm mortar was part of the infantry platoon).
The British Army in @ was using a 51mm platoon mortar up until a few years ago at least, and is now replacing it with a 60mm.