I'm looking to put an additive in my forester's cooling system, I'm not sure where to find out this information.
Either the owner's manual or a repair manual will give you this information.
I happen to have an owner's manual for a '98 Forester, and the value given there is 6.2ℓ, or 6.6 U.S. quarts. The factory service manual for a 2000 Forester gives 6.0ℓ, or 6.3 U.S. quarts. It's possible that the owner's manual value includes the coolant in the reservoir (that's the plastic bottle hanging off the back of the radiator with a vinyl tube running into it from just below the radiator cap).
In any case, the value should be in the vicinity of 6ℓ for any Forester up to '05 or '06 (at least the non-turbos); the engine remained essentially the same over that span, to my knowledge.
The 1998 Subaru 2 litre, and the Subaru 2 litre turbo are listed at 6.4 litres coolant capacity.
You should add your additive/antifreeze at a 50/50 solution with water for protection to -39C. Using distilled/de-ionised water will prolong the service life of your coolant. Increasing the mix with a greater percentage of antifreeze will give you less protection against freezing.