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.