(1) Grab a tape measure.
(2) Walk out to your car and measure to where you think you'd like the wheels to sit.
(3) Order spacers in that size.
You want to do this for aesthetic reasons, which is perfectly fine. But none of us know how you want the car to look, so it's awfully hard for anyone to tell you what size spacer you should get.
Not to mention that there are probably very few folks who have any idea what the size and offset of the Forester STI wheels are. Nor do we know what size tires you're running. Without knowing the key parts of the wheel fitment equation, we can't give you a whole lot of advice.