You need six rectangles of cardboard to form the base of the folding board. The dimensions of the cardboard pieces will determine the dimensions of the folded shirt. 9 inches (23 cm) by 12 inches (30.5 cm) is a dimension that is commonly used in department stores for large shirts. Alternatively, you can just fold a shirt to whatever size that you prefer and use that as a template.
Measure and mark the outline of six rectangles of your chosen size. Then carefully cut them out with your knife. Try to keep the sides as even as possible.