I think any turbo capable of sustaining 20-25lbs of boost on a well flowing 400" motor would be pretty darn big. Steve Morris just made 1600hp with twin 67s on a G3 hemi on E85, I think that's right in @MopaR&D 's ballpark.
So speaking of boost, I believe it is the measurement of resistance. A large port (like Max Wedge size) would take much more volume to create the boost vs a more restrictive head and smaller turbo. Steve builds some crazy engines for sure!