Please see the attached file, care of www.Crutchfield.com
The manufacturers have no real standard speaker size in the 5x7 or 6x8 class of speaker. They really only share the same mounting brackets.
So, regardless of what size you think you have, 99% of all aftermarket 5x7 or 6x8s are interchangeable. So really the argument is pointless.
Crutchfield does a great job of helping you pick which sizes will fit your truck.
Search the forums for additional info on installing other sizes...