It could be a British size. I don't really know exactly what size it is, but a universal socket could do the trick.
But, let's back up for a minute here.
In many circumstances, tightening up the head nuts will not cure an oil seep at the head gasket. This is because there is a "spigot" type joint there, like the old VW engines used to have. This means that if the spigot is a little too tall, it holds the head from squishing the head gasket, and it will never seal then, no matter how much you tighten the nuts.
Next, you should only torque the head nuts to 20 ft-lbs, and not the 24 that the manual says. 20 is all it needs, and that will help save the threads on the studs from pulling out of the castings, and is enough to do the job. If it still leaks after 20 ft-lbs torque, then you need to pull the head and straighten out the situation with the spigot height, put on a new appropriate thickness gasket, and torque to 20 ft-lbs, and it will be fine, assuming that the castings haven't been warped by some previous owner trying to tighten the head nuts. If the head isn't flat, or the barrel isn't flat on the gasket mating surface, you need to skim them just barely enough to get them flat, and then trim the spigot to the proper height to allow the crush of your head gasket.
The spigot mating into the head recess seals the compression, and the head gasket seals the oil. There are two different planes that need to mate-up there at the same time, so the relationship is critical.
Most people here have already been instructed on this, but for a newbie this is critical info.