16x16 would be thee way to go in keeping with the "look" of the GBC Zelda Games. I'm thoroughly enjoying the work you've put out.
You'd definitely have to rework your whole set to accommodate the change to 24x24 sprites.
as an aside, I do like your "mystic" tileset. It was exactly what I needed at the point I am in in my development.
Regardless of your decision, I look forward to see where you go with this set.