Kurt - It kind of depends on what you intend to do with the car, how heavy it is, and how your radius rods are designed. I used 1/2" on the front end and 3/4" on the rear of both my coupe and my roadster. Some people prefer 5/8" everywhere, but 3/4" rod ends will handle about anything you can throw at them. If I had a 4-bar system in the rear, I would have used either 5/8" or possibly even 1/2" as long as it was a lightweight car with small block power (or smaller) and auto trans.

If I knew more details of what you are building, I could determine your needs more adequately. Also, Dave Severson or Brian Ruppnow might be more qualified to recommend something... Tell us what you are building.