I just got everything mounted for my TIPI PEB, so I figured I'd share the parts specs. I bought assortments because I know I'll use them on other projects.
Screws: #2-56x3/8" - I suspect if you used shorter screws, you'd have trouble getting the nuts started at the USB port end of the Pi.
Washers: 2x M3 nylon per screw. This gave me enough clearance, but not by much. I had trouble fitting a simple cable tie under the Pi to help tidy the wiring.