Help

I am about to replace my exhaust on my 91 honda accord from the cat back!  I would like to know what is the size of the diameter of my exhuast.  I don't know if it is a 2 in or 2.25 in.  I know a guy who is selling his muffler.  It has a 2.5 inlet.  What do I need to do to get this to fit on my car?  Is this possible to do? 

You can get adapter pipe fittings at any auto parts store.  A muffler shop would also have the fittings.  Just have them weld the parts together for this kind of setup.