You can add one box dimension and empty box weight to our dashboard.
Note that many carriers do not base the pricing of return labels on the exact box dimensions but rely heavily on the weight of the return. Therefore check with your carrier in detail. Even though you might only be able to set a single package dimension in our dashboard, it might not make a difference with the carrier.
See here how we calculate the total weight of a return: How is the total weight of a return calculated?