There are two write-ups I found on this site, for replacing your knock sensor. One mentions the bolt size as 12 mm, the other doesn't mention size. I just replaced mine this weekend and it was a 13 mm bolt. I figured this out AFTER 25 min struggling with my socket wrench, extension, u-joint and 12mm socket. Once I discovered it was a 13mm, not 12, the project only took about 15 min.
Has anyone else discovered this? Is this dependent on year/model? I wish the how-to write ups had mentioned there may be other bolt sizes. I probably woudn't have messed with the 12mm socket for soooo long. I just figured that I was having trouble due to the poor lighting and extension/u-joint set up.
Oh, well. It worked out fine. It just would have been easier, so I though I'd let everyone else know to try other sockets if you're having trouble.
Have a good one!

 

Mine was a 12mm, my buddy's was a 12mm and my buddy's buddy's was a 12mm. Maybe yours had rust on it or something.

 

It was definitely a 13mm. Maybe it had been messed with by a previous owner???

 

Probably...

 

I can't think of any 13mm bolts on the entire Maxima.

With a 10,12,14, and 16mm socket set, you can almost strip the entire car.