Largest Size Double Gang box

What is the the largest size double gang box for old construction for switches/receptacles?

 

Depends on metal or plastic but very close.

The metal box listed below is gang-able. You can connect two together for a two gang box.
Ultimately there will be slightly more working room in the plastic box.


Single gang 3-1/2" deep cut in gem. (18 cubic inch)
Two gang 34 cubic inch plastic box

 

I need metal because the existing cable is the flex/metal-bx type. Just confirming that I can't use plastic with that type of cable? So basically the width is fairly consistent but I can increase the depth only?

I'm surprised no one makes bigger boxes. I've come across this problem many a time where there simply are two many cables in single or multi- gang boxes especially in older houses. You would think it's easy enough to design boxes that are wider - "step outward" as you go inward from the wall where they would be concealed and yet accommodate normal faceplates.

 

There are larger boxes but they aren't considered old work.

You could use a deep 4" square box with a mud ring but that would require opening the wall.