I want to customize the packaging for the product, but I don't know the appropriate packaging size, could you give me some recommendations?

Sure, could you advise what kind of products you want to pack inside the box? It will be helpful if you can send me some photos or give me more details of the products, then I will give you some suggestions for the package box, like the constructions, dimensions, inserts, etc. 
Do you have the product on hand? It'll be much helpful if you can send us the products, so that we can make a mock up sample of the package box for your checking.