From Kyle Kozubal:
Front Door: 6 1/2"S with a depth of 5 7/16"
Cab Corner (rears): 5 1/4" with a depth of 4"
From Victor Carrola, Jr.:
The Quad Cab uses the same speakers front and rear, 6 1/2"
From Wayne Allewelt:
If you don't have an adapter kit for aftermarket speaker you will have to make your own speaker tabs to fasten the speaker to the door. I found this out the hard way but was able to make some strapping work for anchoring the speaker to the door. The factory speakers are fastened to the door by three metal tabs, that are part of the metal speaker frame. these tabs actually increase the diameter of the speaker mounting locations.