Exhaust systems rarely fit straight out of the box, and that goes for all cars. I have never seen dimensions for muffler systems, either. I'm sure the manufacturers build their pipes to a 'standard gauge' or checking fixture, but normally there is so much room under the car that the pipes can be off a little.
To answer your question, simply center your pipes in the space provided. The pipes are made of bent sheet metal tubing that is seam-welded, which is notorious for dimensions that are all over the map. This is mainly because of the different steels used in the same benders. Each gauge and steel formula springs back at different rates, making bends not so exact. Add engine heat, and tubing moves again.
I've seen muffler men custom fit exhaust systems over here. Many times they take a new stock part and heat it in areas with a torch to tweak the bends. I've also seen them tack weld the joints in strategic places, so they don't need to depend on the muffler clamps to hold a joint from twisting or telescoping in/out.
Previous posts say their system rubbed the steering or the body or whatever. The fix is usually the same, a muffler man tweaks a bend and all is right again. The whole muffler system depends on good solid hangers and well-positioned pipes. There is nothing high-tech here but the muffler guys make it look easy. Just make sure nothing rubs. - Dave