I've been asking around for a while now but even workshop pros couldn't answer this (by citing exact figures):
How big are the brake cylinders of disk and rim brakes in terms of volume and surface area? Specifically the Shimano RS505/685, SRAM Force and Magura HS33 R (FIRM-tech).
EDIT I intended to build a full custom solution working around some of the limitations given by the brake manufacturers.
In short, shifters from brand A on brakes of brand B. I went a different path so I'm not limited by this question any more.