What is the standard diameter for BMX brake mounts
I measured several sets that I have and got the following results. These were used with Shimano, Tektro, Avid and one with "no name" brakes.  This not to say that some other brands don't have a unique size. The male threads that go into the frame were either M6 by 15mm long or M8 by 10mm long. The shaft or studs (part labeled as C) were all just under 8mm, 7.93, 7.90, 7.89 etc. The threaded female hole were all M6. The M8 by 10mm posts all came off of rear brake mounts. The use of the larger diameter and shorter length may be due to the smaller diameter of the seat stay tubing.