Nope, you don't want US Quarter mechs. No tokens are the same size as a quarter, specifically so that they can't be uses in place of real money in vending machines and the like. Standard (non-electronic/comparative) mechs are available in token sizes.
As for what size, I'm afraid they come in a variety of sizes. Chuck E. Cheese, BYOAC, MAME and Retroblast! tokens are all .984 of a US inch, so I'd reccomend you get .984 mechs.
If you're wanting to get a bunch of random US tokens as well as the BYOAC ones,
www.tokenwarehouse.com will sell you a batch in a specified size for a good price, and the owner is an arcade collector to boot.