I usually prepared the black paper in A6 size (a quarter of A4 size about 15cm long, close to 3R photo size), and I will put the artworks in a same size light yellow based background paper in a clear plastic bag to put in my artworks.
I set my artwork in this size is because that was easily for the guest to put in their pocket. Such size is also about a size of our palm; when you hold the artworks on your palm, you are able to view the impact of the artworks.
To make my artwork able to give such feel, I always try to made use of the full area of the paper, thought this could be one of the way to show your scoring skill and how good you can control your hand to produce what you see, especially we are doing a show in the events to catch crowd.
Expect special case like client asked my to produce a smaller size for their own framing size purpose, else I do not do it in such small size by using the A6 size paper, because it will looks ugly once you put it in the plastic bag with an A6 size background.
Art is nothing right or wrong, but in my view, doing silhouette cutting in an entertaining purpose, the art piece size should be in certain visually comfortable size, else that will be too weak for such presentation.