in power store, the dimensions can not be set on product basis.
the dimensions are set in the code to be 6 x 6 x 6.
those dimensions willl be used for all packages, it is not supported to use a product based dimension for the package.
if you want to change it to use a different dimension, you will need to edit the $BoxSize variable in the webassist/plugins/shopping_cart/calculate_shipping.php page