For Sinking leaders 4' to 7 1/2' foot lengths are common.  If the leader gets to short, it will be harder to cast and, in clear water it will spook fish. If the leader gets to long, it will not sink at the same rate as the fly line. 

There are three common styles: knotted, knotless and braided. Knotted leaders are mostly hand tied using ever smaller diameters of leader material to get to the desired tippet size. Knotless leaders are extruded and the sections are formed and ends are sized during the process. Braided leaders are two piece. The butt and midsection are braided from a fine monofilament and each end is generally looped.  Braided leaders do not sink well and are often used with short sections of weighted fly line between the braided butt and tippet or used with split-shot.