I've never built an insulated firebox, but I don't think people insulate the door, do they? If you insulate the door, you're going to have to seal off the intake from the insulation, otherwise you could just have your intake in the door straight in.
If you use the pit calculator on here, put your numbers in for the firebox, cook chamber and chimney sizes and it will give you the exact throat size you need.
@hogaboomer I usually do not insulated my firebox doors especially if there is an air inlet in the door.
Biggest reason is the doors tend to twist or warp.