Length will depend on how thick the board you use. Very important, you should only use corrosion resistant screw. Regular steel wood screw will rust, so you need to get weather or corrosion proof version such as, stainless steel, titanium, brass (steel screws can discolor oak and other woods), bronze, monel or silicon bronze may be used, or a coating such as brass, zinc or chromium applied. Electrolytic action from dissimilar metals can be prevented with aluminium screws for double-glazing trac