The spell Entangling Staff (Spell Compendium p.83) causes the caster's quarterstaff to grow vines and constrict opponents. The relevant section reads:
If your grapple check succeeds, your quarterstaff's vines that can constrict one's foe, dealing 2d6 points of damage
Normally, constriction damage changes with size just like other natural weapons. I am unsure how it would play out in this case, though. For example, if a large hill giant druid uses this against a player via his large quarterstaff, would the constriction damage be 3d6, or would it stay at 2d6 no matter the size of the caster/quarterstaff being used?