Always use two studs on each side of the opening. The first will be continous from the top plate to the bottom plate. This is called a king stud. The next, jack stud will be cut to the height of your door plus 2-1/2" and minus the thickness of the bottom plate 1-1/2" as it will rest on top of this. So your jack stud should be cut at 81" for a 6-8" tall door.
Your header size is determined by the load it carries. If you are building a new bearing wall or cutting in a new doorway be sure to check your local building code span charts or consult an engineer to properly size the header. Once you know the depth of the header, it's width is cut to fit from king stud to king stud, with the jack studs fitting under it to help disperse the loads. A typical header width with single jack studs is cut 3" larger than the rough opening. An example: A 36" door has a rough opening width of 38" so your header is cut at 41". Also be aware double or triple jack studs may be required based on the opening width and the load the header carries. Be sure to check code requirements prior to framing.
You may also want to watch our BUILDING A WALL video