Using this tool will calibrate your project to scale as long as you have a known measurement or at least an idea of what that might be.
1. Hit ctrl-A on the keyboard first. It will highlight everything in pink (Means its selected). Then go to edit >> transform >> scale selection. Mouse cursor should be a plus sign now.
You can choose to turn on your snap tool for more precision or you can just eyeball it.
2. Go from one endpoint to another endpoint on that known measurement and a box should popup asking for the distance. Type in the distance.
3. Then make sure you set your page size and scale in the top menu bar. You can hit autoscale so that it gets placed on the page or you can drag it to the page and center it.