That has been my experience with the fly size/water and Atlantic salmon, big water/big fly, low water decrease your fly size. Or ty them on the same sized hook, but shorten the body well ahead of the point of the hook. This is called tying "low water" or "short taker" here at home. The rivers here are dropping really quickly, right now we have mid-August conditions. Not good. 8's, 10's and 12's are the order of the day now, or 4s and 6s tied short taker. We need rain, and lots of it.
Good luck with your trip, and let us know how you made out.