They should be a 6mm bolt. They're the same diameter and thread pitch as the valve cover bolts, so just bring one of those in to a fastener shop and get a couple of grade eight bolts that are three or four inches long, that'll do the trick all day long.
theres no WAY that they are 6mm bolts, i can guarantee you that. im going to go check right now.
EDIT; yup, iim right... again. i took a 6mm bolt (10mm head) and it slips in w/ no thread engagment. i tried the 8mm bolt (12mm head) like i previously mentioned, and it threads in. grab a pair of long 8mm diameter bolts and run them into the pully. youll probably want to grab an 8mm thread chaser from all the dirt and grime coating the threads inside the pulley. if you dont, the bolt will gall and probalby seize and possibly shear off... then youll be up shit creek w/o a paddle.
I just used an old 2x4 in one hand and a dead blow hammer in the other...And my father just turned the crank 180 every few blows. This only works if the oil pan is off and the engine is out of course.
I wouldn't suggest doing it this way if you have a good engine and just need to change the timing belt or something because you may cause excessive play in the crank bearings etc. I simply used this method because I didn't have a puller and I knew I would be rebuilding the motor.