you can cast the enlarge/reduce spell on yourself..., using only the spell’s enlarge option.
A Ring of Spell Storing can store any spell cast while holding the ring, but only preserves part of the qualities of the spell (doesn't appear to maintain the restriction on casting, emphasis mine):
Any creature can cast a spell of 1st through 5th level into the ring by touching the ring as the spell is cast. The spell has no effect, other than to be stored in the ring...
While wearing this ring, you can cast any spell stored in it. The spell uses the slot level, spell save DC, spell attack bonus, and spellcasting ability of the original caster, but is otherwise treated as if you cast the spell.
Can a Ring of Spell Storing holding an enlarge/reduce spell cast from Duergar Magic be used for the Reduce effect? Can it target someone other than the wearer?