Attribute sizes for shipping - how do you handle?

Looking for ideas or plugins that may help with a situation we have.

We have products with attributes that significantly change the size of the shipping box needed.
In Attributes we can adjust Price and Weight but not Box (length, height, width).
Has anyone else overcome this restriction somehow, or know of a plugin or solution?

For this particular client, choosing a 'king size' product puts it into the over size rating for UPS/Fedex.
So there is a significant change in shipping cost that can vary greatly depending on the destination.

The alternate to using regular Attributes (which only have weight) is to use attributes of type "Associated to product".  Then, be sure your Setting is
shippingsettings.considerassociatedproductsdimensions  True

You can make that other associated product 'not purchasable' on its own by not selecting the "Visible individually".