From: Copenhagen, Denmark
The basic unit is what the game calls an element: battalions, batteries, squadrons. These are the ones you buy in the replacement screen, to replace losses.
When constructing new units, they seem to come in all shapes and sizes, from single-element units, to division-sized units, with several combinations of elements. After training the units, you can combine them into a division, led by a general. You can break the divisions up later, or add more units to it, if you want.