Apple doesn't seem to have any problems making bigger or smaller iOS screens when they want.
It's not the size of the screen that matters. iOS screens come in just two shapes: the iPad and its 4:3 ratio, and whateverthefuck the iPhone has.
That's it. Android screens come in every aspect ratio you can think of.
Apple doesn't understand graphics any better than anyone else; their screensaver might not have flickered, but the Mac is a poor relation to Winders(TM) for graphics performance that actually matters, to wit: 3D. Doesn't matter whether it's an NVidia card or an AMD, on the same hardware, the Windows driver will extract more performance than the Mac OS driver. Saw it myself when I bootcamped my iMacs.
As for nav in the car - I think a tablet is too big for something that the driver will use (although for a passenger navigating for you, they're ideal) and you're probably better off using the satnav and offline maps that comes with your phone (Lumia owners are smug at this point; worldwide offline mapping, free!). It's a better size for mounting on the dash. A tablet would be very distracting.