How to create a space to exact specifications (wall length, wall height, etc)

I am trying to create an exact replica of an existing floorplan. I have the actual dimensions of the space, but I am struggling to understand how the number that shows up as I drag a wall longer corresponds to real life? Is it…meters? feet? something else? I guess I’m really asking about the scale of cospaces & how to lock it in? Does that even make sense? Any help appreciated!