What about the Dominion Planetary Assault Ship (DPAS)? The Defiant was build to defend against the Borg and took everything the Borg threw at it in First Contact (though it was left adrift), but against the DPAS it was blown to pieces. We've seen what 8472's large amounts of energies have done to the Borg.
Dominion Planetary Assault Ship - 4500 meters long (approximate)
Borg Assimilation Cube - 3040 meters (approximate)