Wondrous Item, rare
This is an ordinary arrow while held, carried, or otherwise motionless. When fired, the arrow’s size becomes Medium when it is 5 feet or farther from the creature who fired it. It deals 8d8 piercing damage on hit, and if the target is Large or smaller, the creature must make a Dexterity check or be encumbered by the arrow and move at half speed. The arrow counts as magical for the sake of overcoming immunities and resistances.
I love silly items like this. There aren't enough fun magic arrow types to be found. With that said, did you mean "Dexterity check" or "Dexterity Saving Throw" to see if the target becomes encumbered?
What happens to the arrow after, is it permanently large?