As nobody else has come forth.....Perhaps I am wrong, and those that are the all knowing will jump in, I hope, but; pretty sure it's a 17mm...Take a 15mm with you when you crawl under there but I kinda think that's right...........
I hope you want the size of wrench to remove the plug. If it's the actual size of the bolt/threads etc. you are kind of out of luck tonight........God bless......Dennis