3 1/2" outer diameter washers with 1.5" inner diameter.
All have their benefits. The larger sized washers have a heavier weight, however the extra size of the washer compared to the size of the hole/cup that you are targeting can provide a more challenging game. You decide! I personally prefer the mid-sized washers (2.25" to 2.5"), but I am used to the St. Louis single cup boxes. For my 3 hole washers game I prefer the larger 3.5" washers size for more of a challenge to get it in the 4" cup. Kids generally do well with 2" washers.