I disagree entirely with the inspector.
He said the bottom line is you need the cu. in. volume but you can change the size of length, width or depth as long as you maintain the minimum cu. in. volume of 540 cu. in. I told him we wanted to use the 12" X 10" X 8" box whcih equals 960 cxu.in. and he said the 12" X 10" X 8" box will meet NEC. I told him the wire sizes we are using and he said that we will not have problems with the 12" X 10" X 8" box.
The minimum size is 24"W x 7.5"H x 4" deep.
You can not put a 3" raceway in a 3" box and you can not make the minimum dimensions smaller by increasing the depth to keep the cubic inches the same.
The only way to use a smaller box is to use smaller raceways.
It sounds to me like the inspector is not used to sizing cans with raceways.
All that aside if he is going to approve it, I guess the NEC does not matter.
I would also be surprised if he understands 250.122(B)