Can't remember the exact size I used but they were M8 bolts, i'd say you want a minimum of 20mm long. Just go buy anything 20mm orlonger then you can cut them down to size. I don't know if it matters but I jacked the car up and suspended the whole front end off the ground with 2 axle stands, I would imagine that the turrets would be in a 'slightly' different position if the car was sat on its wheels, especially if only one side is jacked up at a time, probably doesn't matter but it's just a thought and it's the way I marked / drilled my holes in the turrets.