There are many sizes of piping bags and the size depends on what you’re using it for. But, if you’re mostly frosting cupcakes, decorating cakes, or making French macarons then a 16-inch or 18-inch piping bag is best as it will fit about a batch of frosting or macaron batter in it. If you’re decorating cakes with small amounts of icing like writing or small piping details, then opt for a smaller bag like a 10-inch bag.