What is the opening size I need to cut when installing a prehung door on a wall?

Well it depends. Ideally you bought a prehung door that you can slip into an opening that is a little bigger than the door frame. Then you shim in tight to center and woodwork covers it up. If you have not framed in an opening for the door, you'll need to do more work, as the door needs to be secured to stud framing.

Here's a basic link to the general install. It assumes you already have the door area framed. If you need help with this, ask.

http://www.ehow.com/how_15280_install-prehung-door.html

If Your Door Is 24 Or 28 , 32 ,36 All You Do Is Add An Extra 2 1/2 Inches, This Will Be Your Rough Opening, Nichols Trim