Take a look here
for a detailed thread. The basic idea is that you want the rear only slightly smaller than the front. Absloute size of the swaybar is quite secondary, espically on a car with double-wishbones.
You have two aveneues.
The first is the Si rear 13mm swaybar. Handling will improve some, should be cheap to find, car will proably never oversteer.
The second is the suspension techniques 19mm rear unit. (or perhaps something more expensive, but still 19mm) Car will oversteer very occasionally if
a) throttle is neutral
b) rear end is light (empty gas tank, no rear passengers, etc.)
c) pavment is dry and grippy
You jerk the wheel one way and then the other hard, quickly - this can spin you with the 19mm rear unit. However, if you do this, you are already in big trouble.
You can enhance or minimize the effect with tire inflations on the track.
The upside is that you get even better handling. I'm happy with mine, but I recommend you autocross it at least a couple times to get the feel for the new setup.