When choosing your dish covers, consider that they are elasticised and therefor fit a variety of sizes and shapes. The size relates as much to function as it does to the dishes you have in your cupboard.
A set is an ideal way to start allowing you to experiment and see what you need more of. If you are considering dish covers for a gift, big sizes are best for people who like to entertain. Small sizes are super handy and much appreciated for everyday use.