You didn't tell us what exactly needs to be optimized.

NP hardness depends largely on this question.

FWIW have a look at "First Fit Decreasing" algorithm, it has a complexity of n * log n and guarantees a solution not worse than 3/2 of the optimum of the "Bin Packing Problem" (even 11/9 asymptotically)

This should be good enough in most cases.

BTW it's not "Knapsack" since you are not trying to maximize any second weight function of the packed items. They have a size but no additional cost value.

HTH :)