If I remember right I think they are 3/8. They are the same nut that is used in several places on the car. The ones that hold the seat in is the same. I took one off my '93 parts car for my exhaust.
I have to tighten mine like every other week because for some reason the one on the passenger side doesn't like to stay tight. Everytime I pound the pedal down for any period of time it comes loose a bit and I get a nasty exhaust leak.