I'm pretty sure all of the models, at leat until the newest are 6.5 up front (with a tweeter in the door) and 5x7 in the rear. Like I said I can't be sure after the 2004 models but you're safe with these sizes in a 2002.
i'm certain on the front size being 6.5, but I plugged in your car at circuitcity's website and they're saying 6x9 in the rear. I own a 2000 xterra and I've never checked out the rears but from memory, I was thinking they're 5x7. It would make sense that they'd be 6x9 though so call a car stereo place near you and have them look it up. It's def. 6.5 up front with a tweeter though.
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