For the marshmallow top, you can use any size marshmallows you'd like. You just want an even layer that covers the whole dish. I suggest cutting large marshmallows in half, or even in thirds if you are using those. Watch the marshmallows very closely when broiling, they are known to catch on fire if you don't pay attention! Take them out when they are just barely browned. Browning them to a campfire roasted color will make them slightly chewy in these bars.