What size beam to I need for a 14ft sliding door opening in an exterior wall?
An exterior wall is almost certainly load bearing in any standard-design home structure. So yes, you will need to compensate for that if you are opening up a section of the wall.  Considering that you intend to have an opening (14') in a 17' wall, which comes to more than 80% of the wall's length, the beam is going to have to carry nearly the entire load, which makes it imperative that you get it sized and installed right.

As far as beam sizes...you really need to consult a structural engineer as there are many factors to consider and the last thing you want is for your changes to compromise the integrity of your structure.