They are a 1/16 thick IIRC with a 1/2 inch hole in the center. They are about the same diameter as the cap. They fit right in.......What size tire are you running? 35's require 2 washers under each cap. 33's require 1 washer. I just did this before blazer bash.
33's or 35's shouldn't require any washers.
I've run 37's and 42's on my D60 and I've never had any problems, I'm not running any washers. I've had my current 42's at 60+ and I've never had any DW.
IMO, washers are usually a bandaid for some other problem.