5 1/4 round or 5x7/6x8. The 5x7 and 6x8 both use the same mounting holes so either will fit. If you use the round you probably should use an adapter to fill up the empty space around the speaker. You probably also have the tweeter holes in the front doors so component systems are also an option.
6x9 does not fit, but the 5x7 with A 6X8 foot print will. You can also opt for just 6x8's.
I installed 6x8 Infinity Kappa's with external crossover in the front, and Polk Audio 6x8 2 way in the back doors.
Has anyone heard of or used Audiobahn speakers? They have a AS57PV that is a component speaker that you can remove and remotely mount the tweeter. I was thinking about these for my new 06 F150 and put the tweeters in the seperate holes provided in the door and plug the holes in the original mounting bracket. Is there any advantage to doing this or just leave them as is?