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The best sizes of a box must be chosen depending on the real dimensions of the object to be contained in it. If you want to be sure to have perfect packaging sizes you can just measure the product you need to fill in. Take note of its three dimensions (L, D, H) and add 1 or 2 mm for each side to obtain a comfortable package.
For example: if you have an object that measures 100 x 100 x 100 mm its packaging should measure 102 x 102 x 102 mm.