[quote=RickF] Is the Canadian and U.S. quarter the same size? /quote]

Close enough that they seem to be interchangeable up here although some vending machines may have problems.

U.S. quarter is 24.26mm dia and 1.75 thick
CDN version is 23.88mm dia and 1.58 thick

The only thing you really need to fear is the dreaded Poppy Spy Coin.

http://www.foxnews.com/story/0,2933,270413,00.html