^I think it's an 8mm hex. I had to use vice grips on mine to get some leverage, but I also sprayed some PB blaster on there. That might help you out as well.
After pulling the throttle body off, I disconnected the water line that connects next to that plug to get more room. Be warned, it will drip and if you touch any water hose it will squirt, but it's SOOO much easier to get in there once that's out of the way.