At least twice the thickness of the board - with your 22mm I'd be looking at 50 or possibly 60mm long (if 50mm aren't available).
You only need one nail per rail in each board (that's 3 nails per board with 3 rails). With a 25mm overlap you nail 32mm from the thick edge so the nail only goes through one board but holds the thin edge of the previous board in place.
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