If I target Zada with confront the unknown, with additional creatures X and Y in play, what are my choices regarding how big each creature gets?
When Zada’s trigger resolves, you can stack the copies targeting X and Y in either order.
Assuming you control no clues already and that you don’t sacrifice any clues between copies of Confront the Unknown resolving, then you can have X get +1/+1, Y get +2/+2, and Zada get +3/+3 or you can have Y get +1/+1, X get +2/+2, and Zada get +3/+3.
The original copy of Confront the Unknown that targets Zada will always resolve last, so she’ll get the biggest buff.