Products now have dimension fields that any module can use. I think it depends on the use of different shipping modules how you want to interpret them. I can see them used as the actual size of one product, the dimensions of the box several of them ship in. The values of those fields are only important to the CanadaPost module, so far. The only other place they are used is to provide default values for packages in the shipping module. It might be good if the CA Post module could provide some instructions by the product dimensions explaining what it wants.