It is a 6mm stud. Before I mounted my engine, I replaced all the motor mounting , exhaust and intake studs with high automotive studs which I found at a local hardware. The threads on the engine side of the studs were too long. I measured the depth of the threads of the holes where the studs fit into. The thread depth is less than the total depth. I then ground down the stud to match. Since the holes are "blind" that is, closed at the inside end, It might be possible to damage the casting by overtightening the stud if not sized properly.
To use an easy out. first use a center punch to mark the center of the broken stud. The drill a hole with the drill size indicated on the extraction tool. After drilling, insert the tool and turn counter(anti)clockwise to remove the stud. If the stud doesn't come out and you break off the easy out, you will have a bigger problem. The extractor is too hard to drill out with a regular tool steel drill bit. I removed a broken off easy out for a local Mercedes mechanic by using a diamond end mill in a little air powered die grinder.