I believe it is a 14mm. It's not very big, but has a lot of torque on it. Put the bike in gear, preferably 6th, then have someone stand(!) on the rear brake pedal. Then useing a large breaker bar, or a torque wrench with an extension, slowly rotate the bolt counter-clockwise. Be careful of whacking bodywork, yours and the bikes!
I could be wrong, it might be either a 17 or a 19mm, but I believe it's a 14.