7" (and some 8") screens fit a double din opening with a little modification in the back. If you can accept that you might have to visit a junkyard to find an original dash when you sell the car, then things get easier. I have a 7" Xenarc in my Maxima. Looks factory to the untrained eye.
I also have a Maxima and I am about to setup the screen in the dash... could you show some high resolution pictures of your in-dash screen setup so that I can see the result of you installation?