I'm not a very knowledgeable plumber, nor someone who does a lot of plumbing.  As a homeowner, I have done some and will certainly do more, and pipe wrenches are an essential part of a basic plumbing tool kit, even for many simple finish plumbing jobs.  Advice I have found online and gotten from people with whom I work is that the 14" size is the most generally useful, so this is a good first wrench.  Or better yet, buy a pair of them if you are assembling threaded pipe.  However, Channel lock pliers and vice grips are also quite useful, and in many jobs, more useful than a pipe wrench.  Nonetheless, as I said, pipe wrenches are still essential plumbing tools.  From experience and advice, I have decided to get 6", 10", and 14".  And possibly, if I ever need one, I will also buy an 18".  Keep in mind that these WILL leave teeth marks on whatever you are gripping.  If this is unnacceptable, you can possibly put something between the jaws and the object (as the cost of a less secure grip).