Are you talking about just practicing in general or setting up for quilting a project you have been working on. If it is for general practice, all the above answers work. However, if you are asking about a test run for a project you are going to quilt then it is a different story. The test run should be done using the exact fabrics, batting and thread the finish project will have. This way any tension issues or thread issues can be addressed before you start the quilt. The sandwich should be big enough to be able to do at least one complete set of motifs you will be using. This way it will give you a chance to change color of thread or even batting and or backing if you are not happy with the way it looks.