My module allows for a volume value (cubic inches) which is separate from weight.

I have a test version of my module for dimensions based on volume.

What you can do is set volume high for items that are oversized. Then apply a set of dimensions for orders above volume x that allows you to qualify for balloon rate.

If you are interested in trying the test version of my module send me a pm with your email address. Just don't apply the test version to a live site. Use a test store for this sort of thing.