I just installed a set of these on my dad's van two weeks ago. Jsweezy is correct that it comes with clip-on nuts (called speed nuts, j-clips, and other names depending who you talk to). The bolts are not terribly big, but do the job. I don't know the exact size, although the bolt head takes a 10mm wrench (so 8mm bolt, maybe?). You'll need 14 speed nuts and bolts. Each bracket takes one up in the frame and one at the body bottom seam. I've attached pics of the actual OEM bolt and speed nut that came with the running boards (I had one left over not sure why) as well as scans of the installation directions.
You can buy these at any good hardware store, NAPA, etc. The OEM ones are galvanized. Here's an example on Amazon: http://www.amazon.com/Help-45414-Spe...iglink20368-20