This recipe is for Jennifer.
I have included this recipe just in case they should ever, heaven forbid, decide to take the recipe off the marshmallow package.

Melt 3 tablespoons butter or margarine in a large pan over low heat.
Add 40 large marshmallows and stir until mixture is melted and smooth.
Stir in 6 cups of Rice Krispies until lightly coated.
Press firmly into 9x13 pan, cool and cut into squares.

I found this information on the back of the marshmallow package:
5 regular marshmallows equal ½ cup
1 regular marshmallow equals 10 miniature marshmallows
That means you would have to count 400 miniature marshmallows to make the above recipe.
I would suggest using the regular size or just use 4 cups of miniatures.