Everything in box2d has to be a fixture - a two dimensional shape (although single planes are possible in the library, I couldn't figure how to implement them in c2). So, to make lines that have width, you'd have to write that using events - I'll have a think about how that could easily be implemented in c2, but it'll be something like: with a line from x0, y0 to x1, y1 you determine the gradient and then the normal angle, then create a fixture with four corners positioned at the ends using geometry.
Thinking about it, you could also do something similar with the standard physics library - create a rectangular sprite, size it to your line shape and then enable Physics. The challenge there would be to join multiple lines together - once you have joined more than a couple of objects you typically get joint fighting, which is a feature of all physics libraries.
My Box2D+ has a unique 'assimilate' joint which allows you to join objects together by making one a parent and one or more others its children, with the shapes given to the parent body as new fixtures. The advantage is that there are no joints to cause fighting. Here's a Box2D+ assimilate joint demo - you can't do that with the normal physics engine..