I just replaced my calipers yesterday and they were 8mm allen. However my new calipers (Raybestos) had a bolt head instead of the allen key, so there are differences. I don't know if my old ones were OEM or yours are or what,... but 8mm should most likely be what you need.
I'd suggest getting the 'socket' kind that goes on your ratchet handle. Those allen bolts can be pretty tight and can need a lot of torque.