View > View 1:1 > Calibrate 1:1

Lets you set an orthographic view so that your model shows up at true scale (1:1).

True scale (or 1:1 scale) means that a 10 centimeter long object appears as 10 centimeters long when measured on the screen with a measuring tape.

Options

Reference Length: The world space length (in current linear units) of the distance measurement as it first appears on your screen. Choose between 1, 10, 100, or User defined, to adjust this value.
User Defined Length: If Reference Length is set to User defined, this field allows you to enter any value for the length of the initial distance measurement.
Displayed Length: True length of the distance measurement as measured on the screen with a measuring tape or ruler.