No they are the same size, next time you see the speaker box, it will say for 5X7 or 6X8 depending on what car you drive, I beleive in a mazda it is a 5X7 and 6X8 in a ford. there should be a template in the box so it will fit either size, but basically it is the same speaker size just for different applicable cars.
If you ask for a 5X7 or a 6X8 at your local audio store, you should end up with the same size speaker.
Another option is to buy 5 1/4's, which will fit easily. All you have to do is cut a piece of 1/4" hardboard in the shape of the old speaker and cut a hole in the middle of it to set the new speaker in. There are far more choices in 5 1/4", which is why I did it. I ended up mounting Focal Poly-Kevlars in a 136mm (5 3/8") mid in my doors. They fit like a glove and sound badass. I just made another adapter to mount the 1" tweeter in the Mach 460 pillar pods out of plexiglass. Just my $.02. Good luck!
I didn't want to buy another set of 6X8's so I just put my 6X9's in the back, made new holes and bolted them up, I love the bigger speakers back there! I'm sure it isn't too big of an improvement in sound though!
hows that i have 6x9's but they wont fit in the rear, i tried to fit it but couldn't so i just used them as home speakers. i didnt even try to fit the front ones. but im getting mixed answers so is it 5x7 or 6x8? the metal part in the back woudn't even fit the 6x9 i have right now.
the factory opening will fit 5X7 or 6X8. With an adapter plate you can fit all kinds of speakers. They have adapter plates for 5X7 to 6X9s, and more importantly... 5X7 to 6.5"!!! Round sound is best. That's your best choice.