The stock speaker size for the Explorers are 6x8. There are many different choices for you:
Infinity 5x7/6x8 80-20k ($59-99)
JBL 5x7/6x8 50-20k ($79-119)
Pioneer 6x8 35-25k ($79-149)
Polk 5x7/6x8 60-22k ($69-99)
* The price ranges are to the fact that there are several models.
You can also make some modifications to the door to accept a different speaker size such as a 6.5,etc. However, this would probably require sending it to an audio shop, which would be a waste of money unless you are going into a sound-off competition.
The bottom line - I'm running the Pioneer 6x8's in my truck and I am very satisfied. It has the largest range compared to the other speakers. In addition, it doesn't require any mods. or outside installation help. You can do it yourself.
Hope this helps!