As I recall they are 10 mm. Note that there are 15 bolts securing the valve cover. 9 around the perimeter and 6 more under the plastic cover where the spark plugs are located. If you just torque the 9 around the perimeter you may cause the valve cover to bow and create a leak.
See step 10 and 11 in this procedure for the proper installation and torque specs for the valve cover: