Objects are measured in drawing units. Before you start to draw, decide what one drawing unit represents in your drawing.
Create your drawing at actual size consistent with the drawing units that you want to use. For example, the objects below could represent two buildings that are each 125 feet long, or a section of a mechanical part that is measured in millimeters.
The only setting that you might need to specify is the one that controls the display style of the units that you plan to use. For example, the Architectural units style displays symbols for feet and inches, while the Decimal units style does not.