I would urge caution about a turbo Subaru engine in our applications. The sources for expertise mentioned above are installing intercoolers on entirely different platforms (Subarus, vans, and sand rails), where available real estate, routing, water and air flow, second rad, etc. are minor concerns. When you adapt this technology to our replicas, be prepared for lots of trial and error, with error being the common thread.
Regarding size, you want the largest housing you can fit. Intercooler manufacturers use creative license when they describe the HP application of their units, such as 800 HP, 1,000 HP, etc.
If there is a way to do it correctly, the Beck guys are the keepers of the flame.
I tried for years to get it done so it was bullet-proof and never really succeeded. You may have better luck. If you do, please keep the group informed. That's how we all learn.