Sorry man, can't help you there with 100% certainty. I think they're 3/8....? I'm probably way off! But probably the best way to get a good ballpark is to check the holes in the straps. Generally speaking, clearance holes are 2 drill sizes over the size of your fasteners.
EDIT with ebay link: This guy incorrectly measured them as M10s. That'd mean they're 3/8 if he was even close on the thread-gauge. And his length is 2 1/2 " converted to US units.