8mm x .8 threads/mm. 16mm total length, with a 1mm thick/15mm wide fender washer. Threaded section of the bolt is 10mm long. Slightly less common in this size as 1 thread/mm is more common for 8mm. If you can't find it and need it in a hurry, you could tap it for a 10mm of the same length and find a fender washer for that. Not sure I'd tap the .8 to 1, may weaken it a little.
edit - also, on my 2013 there is a metal insert in that plastic this bolt goes through like many other plastic body parts. In lieu of buying that again, you might want to increase the width of the fender washer and use some medium loctite.
Thought I'd look it up. The washer that goes into the plastic is Yamaha part #90209-06187-00, and the bolt is #90119-05062-00 (fender washer is on the bolt)
(I wrote the bolt head on this instead of the diameter for future reference... The correct size is M5x.8x16 with a fender washer on it)