You can add one box dimension and empty box weight in our dashboard.
Note that a lot of carriers do not base pricing of return-labels on the exact box dimensions but rely heavily on 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?