I don't know the dimensions off the top of my head, and I'm too lazy to go measure them. I couldn't find the washers we use anywhere except Tractor Supply Company. At least in the one in College Station, if you tell them you're looking for washers for the game, they know which ones. They are bigger and heavier than any washers I could find at Lowe's or Home Depot.
Scoodogg - My group must be retarded because we play the same way your friends do. Once you hit 21, you have to hit the top of the board with your remaining washers, without going in a hole, to end the game. You'd be surprised how many time the game extends because someone misses the board or accidentally throws one in the hole. If you miss the board, you subtract how many points that got you to 21. If you hit a hole, you subtract that many points.
Also, if you bust, our rules are that you subtract the "hole" points from your current score. For example, if you have 17 and hit a 5 hole, you go back to 12.
And then, there are all kinds of other complicated rules regarding covering points, etc.
All-in-all, it's the most popular thing are our tailgates, and someone is playing washers from right after we set up in the morning until we break it down late at night.