1 3/4" without a doubt under normal circumstances.
Even with new work I use 1 3/4".
Only time I'd use anything less would be for exposed ceiling decking where the the nails couldn't protrude past the bottom of the deck.
Try pounding 1 nail through the roof of each size and see which one doesn't go through in an area that is inconspicuous. That's the easiest way to find out your particular needs.