I think it's an M6 and about 40mm long. Best thing to do is to take out the existing one and measure that. Have you checked that it isn't caught in the pepper pot grill. It's curves round and into the main body, so should have caught it.
I hadn't checked, but I eyeballed it as an M6 and found its replacement just slightly too small... To the point I thought an M8 wouldn't fit. I wondered if for some reason it might be imperial... Will check in the grille though!
