There are many different sizes of light boxes, and the size you need will depend on what you are using it for. If you are using a light box for tracing or drawing, then you will need a smaller size so that the light is concentrated in one area. A larger light box is better for photography or viewing slides and negatives.
The most common size for a light box is 18” x 24”, but they can range from 8” x 10” up to 36” x 48”. The thickness of the glass or plastic top also varies, from as thin as 1/8” up to 1” thick. When choosing a light box, keep in mind how you will be using it and what size will work best for your needs.