10 mm... and yep, I know that this an old post. I just changed mine while changing the spark plugs and flushing the heater core. I just bought my 2010 with 128,000 miles. I wasn't getting very much heat so I thought I would get all this done at once since I had the manifold off to get at the back plugs.
For the curious, I am getting better heat now and noticed that the temp gauge is going a little bit higher now. The new stat is 195 degrees. I ran CLR through the core both ways with a pump. I put an old sock on the discharge hose to catch any debris. There was some but not large chunks that some get on YouTube videos. Not sure if the core has been replaced. I'll probably check in the spring.