It is an odd size and very difficult to get to and get out.
It's 7mm and you'll need an allen key as opposed to a allen key socket.
Ibet you refurb these with the caliper on;)
 
Rear caliper?

Doesnt use 7mm allen keys, front does but not the rear

Two 13mm bolts (require 13 and 15mm spanners) for the caliper and two 8mm allen key bolts to hold the carrier onto the rear beam.