Whiteline sway bars are available in a plethora of different fitments and sizes; there's a Whiteline bar for just about every Subaru car and driving style. We advise mild 22mm upgrade bars for most street cars. Too stiff of a bar can seriously degrade ride quality and make the car prone to sudden breakaway behavior.
Note also that we suggest upgrading BOTH front and rear bars at once. Although upgrading the rear only is less expensive, the result is a car with more roll stiffness in the rear than in the front, and you'll give away ultimate grip as a result.