Partially inspired by this answer, what size of an object could you create with True Polymorph if the target was affected by Enlarge/Reduce? And what size would the object be after Enlarge/Reduce wears off?
True Polymorph says (among other things) (emphasis mine):
Creature into Object. If you turn a creature into an object, it transforms along with whatever it is wearing and carrying into that form, as long as the object's size is no larger than the creature's size. The creature's statistics become those of the object, and the creature has no memory of time spent in this form, after the spell ends and it returns to its normal form.
Enlarge. The target's size doubles in all dimensions, and its weight is multiplied by eight. This growth increases its size by one category-- from Medium to Large, for example. [...]
Reduce. The target's size is halved in all dimensions, and its weight is reduced to one-eighth of normal. This reduction decreases its size by one category--from Medium to Small, for example. [...]
The two spells both require concentration so they would of course have to be cast by two different people. The options I see here are
Medium creature is enlarged to a Large creature and then polymorphed into a Large object. Spell wears off and the Large object is now Medium object since it's still a valid target for E/R.
Medium creature is enlarged to a Large creature and then polymorphed into a Large object. Spell wears off and the Large object is still a Large object as it's no longer the original target of the spell since it targeted a creature in this instance.
Medium creature is enlarged to a Large creature and then polymorphed into a Medium object since the creature is normally Medium. E/R ends.
Some option I haven't thought of.