To begin, take a look at the size of the studio. Make sure that your set will allow plenty of room to work around it, and take into account the need to reserve space for camera and lighting equipment.
Consider your budget. Are you able to afford the additional supplies that come with building a large set? For the beginner, it is often practical to start small and work your way up to a larger scale. Keep in mind that larger sets frequently require more detail, which takes both time and effort to achieve. Small sets, on the other hand, can appear rudimentary and lacking in detail. Try and make your main characters large enough so that you can have the option of taking close-ups and creating dialogue.
When you have decided how large to build your set, consider the height and overall size of your character. Make sure that the design of your figure leaves plenty of room to allow them to jump, skip or fall, and give yourself enough leeway to be able capture important movement from different camera angles. Most clay and plasticine puppets work well when they are between 6-12 inches tall. A larger character will give you many more options with regard to detail and facial expression, but will also be heavier and not as easy to manipulate.
After you determine what size your character should be, begin to plan for set items such as furniture, bicycles, cars, etc. If your figure is 12 inches tall, you are working with a scale of 1:6. Measure the width and height of the furniture in your home. Divide those numbers by 6. This will give you the dimensions you need to use in order to create furniture for your set. Continue this process with other items and props you need to set to scale. If your character is 6 inches tall, you will shift your scale requirements to 1:12, and adjust all other measurements accordingly.
A great solution to designing your own set is to purposely create characters which will fit the scale of popular toys. If your character is the same size as a Barbie doll, then you can instantly access a wide variety of props and accessories which will conform to the scale of your production. Likewise, if you make small figures that could fit into a dollhouse, you will be able to choose from a vast selection of furniture and other items from craft stores and online. The large models of G.I. Joe dolls are a great size for a stop motion puppet, and will allow you to use items such as motorcycles and cars that fir your character.
When choosing your production scale, consistency is the key. Pay attention to the details of your set, and know that size does matter!