In ArcSite you have the ability to manage and set a "scale" for your drawing using the Calibration tool. You can manually set a distance in the app to calibrate the entire drawing around. This is applicable both when drawing over existing plans to calibrate that document, and also on a blank canvas to set the scale manually.
Note – Based on that entry, all future objects will default to that scale.
Steps for Existing PDF or Image
Tap on the Calibrate icon in the lower-left corner of the screen
Accept pop up message
Drag measure bar to match known distance
Tap to accept
Starting With a Blank Canvas
Drag across the entire width to define. Note it does not have to be exact, as you are setting an approximate size to match the needs of your upcoming project.