Building Automation J-box sizing for Cables

Good afternoon, was wondering of any specific formula or code item that relates to signal cabling such as Building automation cables for start/stop, status etc for sizing of j boxes.? Such as for risers and in mechanical rooms where all systems must be in conduit.
Currently our engineered drawings say to"provide a plenum rated pull-box on every floor per riser. Minimum size pull-box shall be 12"X12"X6", pull box size shall increase depending on quantity of conduits entering box. Adhere to NEC Guidlines."
Our riser EMT is 3" and the most cables in this 3" shouldn't be more than 6 communication conductors, the conduit is sized for future use. The cables are 24-1P Lo-Cap.
Each riser floor will have said j-box with 3" emt in and out. And a 1" from said j-box out to the floor to above ceiling where we can run open plenum to various controllers.
Basically they are making us do the research for making sure that the 12X12 is large enough and if not we are responsible for finding the correct size(s).
Any assistance would be greatly appreciated.