Here's what the WPA's equipment specifications say about the rack. Nothing is said about dimensions.
"The wooden triangular ball rack is the recommended device to be used to rack the balls to ensure that the balls are properly aligned and in contact with each other. Both surfaces that can make contact with the table-cloth when moving loaded rack to and fro, should be very smooth in order not to incur any damage to the cloth underneath. Plastic racks are not recommended, they are flexible and tend to deform, making proper racking of balls time-consuming, if not impossible."