I just put on a Magnaflow 4x9x14 muffler with 2.25 in/out on my 02 Tracker.  The extra power and torque are really nice, but it's just a little too lould when under load (like going up a hill(on road) etc.)  I like that loud noise, just not so much on my daily driver.

Would a 5x8 oval be quieter than the 4x9?  How much less noise would a 18" make than the 14"?  Basicly, what is the quietest muffler that magnaflow makes, with a 2.25 in/out, that would fit on my Tracker?

basically the larger a muffler is the quieter it is going to be. it also depends on how many baffles the muffler has The solution to your problem lies in what sound you are trying to achieve. for instance I am quite happy with my 2.5 inch with hi flow cat and glass pack which runs as straight as possible and exits right beside the rear bumper. there is not a lot of sound in the interior even under acceleration. If your noise is in the higher frequencies perhaps adding a glass-pack into the system will lower the sound but still leave some low frequency rumble. A resonator will also modify the sound. Dave

You answered your own question.  Find the biggest muffler they make that fits your vehicle, pipe, and configuration and run it.