What size ensign?

[jbinbi]

I have a 3' staff to fly the flag off of, choices are 12 x 18", 16 x 24", or 20 x 30". Figure the goldilocks approach is best, middle one? Really large one seems like a good downwind sail.

[StuM]

It's not the size of your staff, it's the size of your boat (LOA) that should dictate it.
There are various "rules of thumb", but the most common one is 1" per ft.

So 20 x 30 in this case is a bit small, 24 x 36 would be more appropriate.