Speaker sizes

Looks like 6"x8" in both front doors and rear side panels, per Crutchfield. You could also do 5.25" components, coaxials, or triaxials (w/adapter), or better yet even midbass's in the doors with components in the kick panels (w/custom kick panel enclosures). You have many options and a good starting place, depending on your needs and funds:

Steven

My '99 XLT four-door had four 6x8s in it.

If you're going to just replace the speakers, you might be disappointed with the results. I replaced all four of the factory speakers with Pioneer tri-axial 6x8s, and they didn't make that much difference.

Get a subwoofer. Even if you don't turn it up loud enough to shake houses as you drive by, the sound is infinitely better than just replacing the door speakers.

I now have a 2002 EB, and the subwoofer system in that blows the doors off the one in the '99.

Steven is right---the Explorer has a lot of options. Use the doors for midrange and treble, and put a subwoofer under one of the seats or in the rear side panel.

You have 6x8's all around your truck

Mark S Owner SoundsExtreme

Yah, I have Clarion all around. Clarion 7" LCD Touch Screen, and standard 6 X 9 Clarions in door panels. The real difference comes with a subwoofer. I decided to go with an Infinity Kappa Perfect 12", which runs a little pricy at around 200 a piece, but provides great response, and deep base. I love Infinity. If you can afford it, Infinity makes great mid / high range speakers as well, but again they are costly. Any amp will do, I tend to like Kenwood, which I run. Let me know if you have any questions or comments. I have done quite a bit with my explorer and know some "do's" and "dont's." I am a sound quality guy, not insanely loud, inaccurate, and just plain annoying bass guy. Also, I know some great sites to purchase from, that have great prices and good customer support.

SOME PICTURES OF MY EXPLORER HERE. I WILL BE POSTING MORE VERY SHORTLY.