packaging size is a requirement of the shipping rate lookup service.
However, the shipping rate lookup will look for 2 rates:
1) Weight based - it will calculate the shipping for the specified weight
2) Dimensional Weight - It will look at packaging size, and determine the maximum weight for that size, the calculate shipping for the dimensional weight,
Which ever is largest will be the rate returned. In other words, if you enter the packaging size as 1x1x1 it will be the same as if it was not entered at all.