Unit

One unit is a volume of 1x1x1 studs. This is the unit in which everything is calculated, eg. wood selling price, wood required for a Blueprint.

To easily visualize this, set a sawmill to create planks of height 1 and width 1, then cut it with your axe to a nice cube. This will be 1 unit of wood.

A tiny floor is also a 1x1x1 stud.