Bolt size

I did this too. Were you able to get the plate off? Were both stripped? I even tried heating it with a torch to try and break the loctite but didnt want to do that too long since it would heat the MC and possibly damage Orings. After stripping the head I ended up using an EZ-out which snapped naturally.

I believe its M8, but check the parts list. Heres what I had to do, long process but it was necessary:
1 of the 2 bolts were stripped. I ended up draining the rear brake and disconnecting the hose at the Master Cylinder. I removed the entire mount with Master attached. I carefully clamped the master in my vise and with an upward motion on the heel plate, rotated it which was enough to break the loctite or whatever was holding it so tight in place. Removed the bolt, rode to Home depot and bought a replacement.

I highly recomend anyone to use an impact wrench if you feel that risk of possibly stripping a hex head.

On the R3, what you described gave me high blood and probably a head ache. Realigning the rear brake bracket while trying to get the rear axle back on is lots of fun too if you like to torture yourself. I do all my work by myself, so that doesnt really help lol.