Keep in mind all is relative. A unit means nothing in itself.
But put a 20 units tall mesh in your scene, if your mesh is supposed to represent a house which is 10 meters in height, then you just have *decided* that 1 unit is 2 meters. You just have to be consistent with all your other meshes (a human character should in this case be around 3.6 units in height).
But put a 20 units tall mesh in your scene, if your mesh is supposed to represent a house which is 10 meters in height, then you just have *decided* that 1 unit is 2 meters. You just have to be consistent with all your other meshes (a human character should in this case be around 3.6 units in height).