I have a pair of rotors that look good but I am not sure what size is good or bad as far as thickness goes. what size are the factory rotors new and how thin can they be turned till they are considered illegal for a shop to install them on a car??

Look on the back side of the rotor- Minimum legal thickness is always stamped on the rotors. You'll need a mic to gauge them- however, they are gauged after they are turned on the lathe. You don't know how bad they are and how much needs to be cut until they are turned.

Keep in mind that the 'discard' limit is different than the 'machine to' limit. The number stamped on the inner part of the rotor is the discard limit. This is when you need to discard the drum. The rotor SHOULDNT be machined to the discard size because you have to allow for wear. You should generally add .015" (or 0.381mm) to the discard to get the machine to limit.

Let's say that you have a rotor and the discard limit is 0.965". This means that you can only machine it to 0.980". A good shop should know this general rule and should abide by it. Plus they have to keep it mind how much material they have to take off to take care of any run-out in the rotor and/or thickness variation.

As AGood2.8 said, you will need a micrometer in order to measure the thickness of the rotor.

I am a michinist so I just brought my 1 inch mic home and both rotors are over over 1 inch. 1.010 and 1.020.