There are a couple of different hole sizes in 2 1/2 x 2 1/2" cardboard holders made for chips and coins. You want the smaller one, which fits a chip just fine. The larger one intended for larger coins is no longer made and is out of stock at every dealer I've checked.

Note that a chips will "kind of fit" in a standard 2x2 cardboard holder but the edges won't show. Not recommended.

Here is a scan showing the comparison between the 2 sizes of windows in a 2 1/2" cardboard holder.