I suppose you could really use any size paper baking cups that you wanted to use. You can even use the little foil cups, too, if you prefer.
The size the recipe is actually calling for is the mini baking cups about 1 1/4" in diamater, like the ones listed here at Candyland Crafts.
As you can see, they have a variety of sizes and colors to choose from. You can also usually find the mini paper or foil cups at your local grocery store.
I'm a big fan of the chocolate peanut butter marshmallow cups. I hope you'll love them, too! Tell us what you think about the recipe once you've given it a try. :-)
My mom got me into this one! She made it (with my sisters and I) for Christmas a few years back. I will never forget that day! We had a blast in the kitchen.