For a 10 inch round cake you can use a Large Priority mail Box 12x12x5-1/2 for a 1/4 sheet you need a 12x15x5-1/2 If you cant find this boxes at the post office, you can ask the USP or FedEx people and they will be able to help you. If you cake docent meet this criterias you can try a packing box store or even staples may have a proper size box. Remember that the bigger the box, the more expensive it gets.