When yuo insert or update a product, the Additional Settings tab will allow you to enter the eight for that prodyct.
the box size is set in the webassist/plugins/shopping_cart/calculate_shipping.php page:
if ($WAGLOBAL_Country == "US") {
$EndUnits = "lbs";
$BoxUnits = "in";
$BoxSize = "6";
} else {
$EndUnits = "kgs";
$BoxUnits = "cm";
$BoxSize = "15";
}
for US it will be set to 6 x 6 x 6, you can change that by changing the $BoxSize value.