How big is a potty trainer? Potty trainer sizes differ from one manufacturer to the next. Basically, what you will need to check are the following: height of the seat from the footrest, the height of the backrest, the width and depth of the seat and the maximum toddler weight the seat can support. From the seat to the footrest, the height for one manufacturer’s potty trainer could be between 6 to 9 inches while the width and depth of the seat could be 5 to 9 inches by 7 to 10 inches; while another manufacturer may have a totally different size guide.