change how file size is figured in system 10.11 ?

It might be old and incorrect but if the entire rest of the world still use it, that might be new and righteous, but totally useless. OSX should give users a choice. The "Think Different" must be there, somewhere. Is not "different" by the rest of the world, but different by people who want to impose their will, in the first instance. See the Apple 1984 commercial, and how we got too far from the core spirit...

The rest of the world uses SI units and thus must adhere to using exponents of base 10 if they use the common notation of kilo, mega, giga, tera, peta, etc. You can't just arbitrarily decide that you will use 1024 instead of 1000 when you use the prefix, kilo. If a kilobyte is not 1000 bytes but is instead 1024 bytes, then it is not a kilobyte, it is a kibibyte.

As far as I'm aware, there is no way to change it. Apple abandoned the old, incorrect way of displaying size based on 1024 long ago. There was a third-party utility that did that, switchDiskSizeBase, but I don't know if it works. When Apple required Code-signing, I don't think it kept up, so it won't work in El Capitan or later (unless the author updated his near decade old code).