In a previous project log, I described 4x12 inch vent boxes being supplied by a 6 inch diameter duct. How can that be? Well, the situation is a little more involved. The cut-outs in the floor are all the correct size to fit a 4x12 inch register. Some of the vent boxes, however, are larger than the cut-outs.
Could I then use that larger interior space to use larger fans (and run them at lower RPMs, and therefore lower noise, for the same air flow)? Not really.
Although some vent boxes are pass-through (ducts on two ends), the vent boxes in the rooms of interest for this project are all at the ends of runs. The supplying ducts are indeed 6 inches in diameter, but the vent boxes are tapered from a 6 inch opening for the duct to the 4 inch opening in the floor cut. By fiddling around and putting things at angles, I might be able to replace one of the 92mm or 100mm fans with a 140mm or 150mm fan. That seems like a lot of fooling around for a speculative pay-off. I'm not going to bother with it.
(I'm not certain how the tapered shape would affect my previous consideration of static pressure due to size of the supply ducts. Obviously, 4 inches is less than 6 inches, but it's also some part of the 4x12 inch area. I don't have much hope of figuring that out as a calculation.)