fords and some mazda's have there own speaker size, it's 6x8 (thats why the pioneer's say right on the speaker "custom fit for ford,mazda")
the pioneer 6x8's are a direct fit for fords and mazda's, and for the price are not too bad, is that the TS-A6872R, they are definitly better than a blown speaker, pioneer, sony, and kenwood tend to focus on loudness instead of sound quality, i would recomend somthing like an alpine or infinity for your fronts for better sound and then take the good factory front and put it in the back
you can use a 5x7 if you want to but the size is 6x8, and a 5x7 is a tad too small, most 5x7 do come with adapter rings to fit the 6x8 opening a little better also remember to get the speaker harnesses (metra 72-5600 and you will probably have to ask for them because they usualy keep them in the back) so that you can just plug in the new speakers and dont have to cut off the factory plug (rookie sh!t) and then have to worry about getting the polarity right (because it will sound like sh!t if you hook it up backwards)
incase there is any confusion, Lincoln = Ford