sway bar sizes?
 

can someone enlighten me as to what the different sway bar sizes achieve?

i usually see 32mm as an upgrade for the fronts, but for the rear i see 21mm, 22mm, and 25mm

The bigger the bar the more roll stiffness typical autocross setup is 35mm front and 21/22 in the rear. Some road race guys use a 25mm rear bar but thats with a lowerd panard bar.