In its current form the truss plugin is only capable of generating geometry. It does not attempt to engineer or size the members, that information is input by the user. For engineering of a common fink truss you can try my Truss Designer here (note that I have not even progressed to any other truss type yet due to the complexity and sheer volume of checks required for a typical truss):
So unless you have a rough idea of what your loads are and what the member sizes will probably be or you already have this information from shop drawings of a truss manufacturer then your sizing of the top and bottom chords is a guess at best and may change when the quote comes back from the truss manufacturer.
I think what you are requesting is the ability to make the member sizes parametric so that they can be updated easily within the component at a later time when more information is present about the truss engineering. I agree that this would make the geometry and the plugin that much more useful. However, I did not know that SketchUp has any parametric abilities. I will have to look into this further and see what I can do.