I had this exact problem when I created the Trane VAV boxes for my company.
I wound up having to make 6 part families instead of just three:
VCWF - Round Inlet - Hot Water
VCEF - Round Inlet - Electric
VCCF - Round Inlet - Cooling Only
VCWF - 24RT Series - Hot Water
VCEF - 24RT Series - Electric
VCCF - 24RT Series - Cooling Only
The round inlets include VAV box sizes 04,05,06,08,10,12,14,16. The 24RT series is a lone part family, that includes a lone part size...and the only difference is a rectangular inlet. 😕