A standard Wayfarer tiller is 1400 mm in total or 1300 mm from the front of the rudder. AFAIK there are no rules limiting the length, the diameter, material or shape.
A nice effect is achieved by laminating layers of dark wood (mahogany) and a light wood (spruce). I saw a tiller once that was laminated in a shallow S-shape. It not only looked very nice but it also allowed access to the rear tank without having to remove it.