For the most part Mike, it has to do with your driving style and preference. If you are a rally X racer you would want very small bars or even go without. If you're Auto X though, you can get them huge to reduce the body roll of the vehicle to nil. Looking for balance is what I was after. There is inherent understeer (plowing) with my car (2009 wrx) so I went with an adjustable 20mm in the rear (Whiteline) which will tighten up if I want, and a 22mm front (Rallitek). Honestly, I don't really track the car, but for daily driving street and highway I am really happy with the improvement! If I were to push the driving just a hair more though I would want larger bars. Say 24mm front and 22 rear or larger. As of now I REALLY have to yank on the wheel and get the tires screaming to get some tilt out of the car and that's simply due to the stock shocks from what I can tell.
So google 'understeer' and 'oversteer', or perhaps search the forums (I know boostaddict has a good read up in his massive build thread, but no idea where to find it) to learn what it's going to do with your car. Really large front bar will make you understeer more; Where as a really large rear bar will cause oversteering when your tail end swings out on hard turns. So like I said, it's a balance for your driving style. I hope someone else can come to confirm the above statements and throw out some info links and/or better advice. That should get you started on your search for sway bars though! Don't forget to look at endlinks as well. They are what the sway/antiroll bars attach to and would be desired to upgrade them at the same time as well. C-ya!
Oh, and the effect on your handling is going to be *less body roll from the car = more grip/available tire patch/traction = more control and confidence while turning!
Only use roads to get to the dirt; Then keep it sideways! >;D