How big of a back-horn would be usable for a 4-5" FR driver?

With a large compression horn, its inertance? is so high that room loading doesn't impact the response below tuning and neither does EQing, though the driver will overheat/destroy itself in trying to overcome it.

In short, better to map the room for its gain curve then design the horn's gain BW to mirror it like with any speaker system to get the lowest, flattest in-room response.

GM
 
neither does EQing

You've lost me here.

EQing is just telling the amplifier to put more or less voltage at the terminals at some frequencies, compared to others.
If EQ won't change the response, the speaker must be acting in a very non-linear way, such that increased voltage doesn't change the SPL output.

Since, even at pretty silly power levels, speakers are fairly linear in terms of increased voltage = increased SPL, I can only conclude there's a miscommunication here somewhere.

Chris