Using a 16" main run Vs. a 14" is no problem.The issue may be the size of the ducts. What effect is there on having, for instance, a 7" flexible duct supplying a room that only requires a 5" flex duct? What effect is there in running a supply that is 16" round rigid, when it should be 14" round rigid? My guess is that the air handler is going to have to be larger and put out more air to be able to fill the larger ducts and still deliver the proper amount of air to the rooms. Is that correct? If so, how do I figure out how much bigger it needs to be?
A 7" Vs. a 5" could be compensated somewhat by using a damper at the branch run, & using a diffuser that had the correct free-air (Ak) sizing to provide the proper throw.
Whether it could work properly depends on the entire duct design layout!
A Manual D should be performed on the duct system to see the best available options.
16" Flex Duct 2-Ton Max 900-CFM (one 90ฐell) friction rate 100' .09" velocity 645-fpm
14" Flex Duct 2-Ton Max 900-CFM (same) 0.10" 842-fpm/Vel.
The 16" should work alright; depends on the layout of the duct system.
RD Rigid (same as above) 16" friction rate 0.04" 645-fpm/Vel.
14" 0.09" friction rate 842-fpm/Vel.