Well, first thing, you'd better have a front bar on there already. You never want a rear bar without a front bar, and the front bar needs to be the bigger of the two. Why? Because stiffening the front roll resistance promotes understeer, and stiffening the rear promotes oversteer. More oversteer than understeer is not really safe on the road (although it might be the quick setup for auto-x depending on the car).
So, for a factory front sway bar, which I think is pretty dinky (15/16?), you'd probably only want a 5/8 rear bar. With the typical aftermarket 1-1/8 front bar, you usually use a 3/4 rear bar. Of course, what counts is the actual spring rate, but this is usually referred to by using the bar diameter. Another factor is the length of the lever arm, which can vary in different designs. But most A-body bars are pretty similar in pattern.
The type of bushing you use in the end links has an effect on how quickly the bar starts working. Rubber bushings will deflect a lot before the bar kicks in. Poly is much stiffer. I set up my front bar with heim joints, so there is no delay at all, and I have poly on the rear. But you can use this to fine tune your setup.
