Having used a fair few flemming
wind vanes the exact size isn't critical. It's the balance that IS critical.
I've made a heap of different vanes for mine. I good quality 6mm
plywood approx 9 inches-10 inches x four foot high (easy to get a bunch out of a sheet of ply). Shape the bottom to fit then cut big holes near the top until it balances nicely and returns to upright.
Make it a bit lighter to allow for
paint and moisture and cover the holes with stickyback nylon sailrepair tape, vynal book covering or duct tape.
Then go sailing. Should
work fine, if not modify it.
This size is a pretty good all round and as a light air blade. I carry a couple this size, and for heavier airs a 4 foot × ten inch tapered vane that tapers down to about 2 inches at the top. This size is great from about 15 knots through to 50 knots.
The twinwall polycarb ones are only anygood up to about 30 knots. Then they start self destructing. At least the one I used did. They are also often too light, so they dont
work well in light airs.