Either 12-14mm, I believe the Dax motor I'm running is 14mm. Standard sockets are deep enough, although an extension might be necessary to clear the cooling fins. I always use 1/4 inch drive, to keep me from being over zealous. I don't own a torque wrench, but I'm one of them who can "feel" torque, I've been turning wrenches a long time, and I'm familiar with the sensations of steel preparing to give. There is no fastener on these engines that needs to be that tight. I do not always torque wrench, but when I do, I use the needle type.
Buy one, it'll save your bacon. Most of my repairs are done on the roadside, well away from anyone and anything, crosslegged on the shoulder of some lonely highway. Those situations are rarely conducive to proper equipment usage. After making a head gasket out of a bud lite can with a razor knife, just get the thing tight and get it home. That is not the proper way of things, buy a torque wrench.