It all depends on what you expect the tip plate - winglet - to do. If it is to stop outboard flow at higher angles of attack almost any size will be better than non at all. I think that in pattern work they aren't used because the typical FAI type of pattern ship doesn't do any high alpha work. A plain wing works fine for their maneuvers.

If you are stooging around trying to win the XFC then they will help give an edge with almost any size you put on - the bigger the better like the dual SFG of the Mexican entry ( until the judges start thinking they look funny and get prejudiced against the airplane). After all in a contest like the XFC the airplane spends a lot of time in extreme flight/airflow-conditions. Last year I saw a roll after takeoff as soon as the wing tip would clear the ground. That might have been the last level flight during the whole flight!

If you expect a decrease in sideslip angle in knife edge flight then the big ones that Dick had in the first photo are the way to go. If the full wing will pull 10g or more then the big SFG have enough area and can get enough sideslip angle to fly level flight nicely. But the airplane won't suddenly fly level in knife edge flight if they are small like the other photos.

A nicely designed SFC/tip-plate does look good.

Ben