5" is all you need... I love it and agree with Luke.

 

The bigger the stack the louder it will be inside the cab. When I had my 5" stack in, there was only a certain RPM where you could even hear the exhaust. With the 7" stack, it was much louder in the cab. I personally loved the sound of the stack, but just didn't like the look (I ran it for a year) and now I'm back to a full 5" system out the back.
Basically every exhaust system I've had (stock 3" to 4" tip, 5" exhaust to 5" stack, and 5" exhaust to 7" stack) have all had different sound qualities. Here's how I break it up:

Stack= smooth deep tone (as raspy noise goes up into the air)
quieter inside the cab, but louder when windows are up (drone)

Side exit= deepness, but with all the raspy sound
louder with windows down, as every bit of sound it exiting the side of the truck and echoing back to you

Hope this helps. Remember that I have a Ford cab, and I have heard that stacks are unbearable in Dodges, was not the case with my Ford

 

I had a 4 inch down pipe to dual 8 inch stacks cut at the top of the cab and it was incredibly loud. I actually took them off because I would get a headache from the noise on long drives