HEres an easy one for ya, what size are the speakers in a 91' Ford explorer 4 door?

probably 5x8/6x9, same speaker.

There the typical Ford 5x7 if I remember correctly. I had no problem's making my 5 1/4" components fit in there.

whoops, not 5x8/6x9, 5x7/6x8. Duh.

Yep you can get 5"x7", 6"x8", 5.25" or 6.5". The 5.25" and 6.5" will require a little adapter plate that will fit the stock opening. Then you just screw the speaker into the plate.