I have spent the last few days searching and can not find the answer i am looking for.
Does ANYBODY know what size the piston is in the stock master Cylinder on the 2008 990 SDR??????????????????

Nope.

I looked as well since I was intending to rebuild mine before I discovered what a pain in the ass it would be.

If it's any help Brembo recommends the 19 RCS as a replacement for the OEM MC.