The proportioning valve that you buy is going to dictate the size of your lines for you because it is already threaded for a certain size brake line/fitting. I am sure there is some science to it, but I just bought the size that fitted them and went forward.
The rear brake threads on my proportioning valve (I haven't done the back yet) is a larger size thread, so I am sure that it will match a specific size metal line. The rear brake line does serve both rear wheels - maybe that is why it's larger? (although a lot of people plumb their front lines off of one side of the proportioning valve and use 3/16 for both of the fronts)
