5 1/4" up front. The only ones I found that fit are Pioneers as they have a slim profile(parking break mech gets in the way of most). They will not mount all the way flush on the grills (small central dome protrudes) so you will need speaker baffles to "enclose" the back of the speaker from the front.
Another option, that is slighty more complicated, that will give you better sound would be a composite speaker up front(cone and tweeter can mount seperately). This allows for flush mounting the face of the speaker with the dash. These are more expensive and take a little more time to install.
-I run the pioneers up front, straight to the head unit. The I run 6x9s with a 450 watt amp in the back. An amp is a must to have non distorted sound, that you can actually hear running down the road with no top. That said, in an open air jeep, it seems the sound is still somewhat shallow. If you want truly excellent sound, a sub would be a must. i chose to go without for now at least...
Good info here. I have component Pioneers in the dash of the green Jeep and they sould 100x's better than the two way Pioneers, Polks, JBL, and Infinity speakers I've had in the tan Jeep over the years (the component speakers came already in the green Jeep.
I have 6x9's in the housings pictured above with no external amp. I do run a high powered Kenwood (15Wx4 channels RMS) head-unit though, and it's plenty loud for me.
Ran a generic bazooka with built in amp for awhile, but I'm not THAT much of an audiophile so when it was stolen I never replaced it.
Honestly, having tried Polk, Kenwood, JBL, Infinity, and Pioneer speakers in my Jeeps, the cheap Pioneer 6x9s from WalMart sound as good as any of them for the back, and they have poly cones with rubber surrounds so they'll hold up to Jeep use, even leaving it out in the rain without a top. Paper cones with foam surrounds will disintigrate, but almost all name brand speakers now have poly cones.
I will say I was shocked at how much better the component speakers in the dash sound over the 2 way. The prior owner of green Jeep put the speakers behind the dash in factory location, and mounted the separate tweeters above the oil and volt gauges.