I don't remember.
I think they were 6mm.
You only need to replace one of the studs on the drivers side where it hits the back of the switch.
I pulled the stud out of the drivers mirror, the top one, took that and the nut down to the hardware store and bought several lengths, the left overs are now in my misc. hardware drawer, and I can't remember which one I actually used, I think it was about 1. 5" but metric length.
Many of the weird shorts, fuses popping and mirrors that only go down problems I've read on this very long thread are probably caused by crimped wires during install.
a couple more tips.
#1 if doing this yourself, roll down your windows prior to taking apart your door panel, so you can reach through the opening to hold the mirror while inserting the grommet, installing the backer plate and putting on the nuts.
#2 after fixing the short I caused on the passenger side mirror, I used some electrical tape to hold the wire and foam pad in place while I got the mirror aligned and in place, then yanked it out right before tightening down the mirrors.
#3 buy some panel clips, you will damage a few and need to replace them, our trucks rattle enough you don't want to help it along.