A leader is typically 7.5 to 12 feet long, thick at the butt (where it connects to the fly line) and thin at the tip (where the tippet ties on). The taper is what makes a fly turn over and present cleanly instead of piling up on the cast.
The lighter the tippet, the smaller the fly should be. A 4x leader tapers down to roughly 6lb-test material.