This is not a new question, to this group. The pit calculator is a great tool, I believe the safest bet is stick to the numbers and the fuel is what will be reduced.
Making memories.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
I'm still learning but I think if you reduce the size of the FB you are missing the point of the insulation. The point of the insulation (IMO) is to make the firebox more efficient. If you insulate a firebox it will be more efficient but if it's too small for your cook chamber you wont have the right air/heat flow. I think too small a FB will have bad air/heat flow weather it is insulated or not.