You can buy touch overlays for 12.1" and there are plenty of 12.1" around because it used to be a common laptop size. But you're smoking crack if you think 10.4" is too small! Most people here think 7" is 'big'. I think 8.4" and 10.4" are nice sizes, but anything bigger just becomes a pain in the but to install and takes up too much space in your dash. There is nothing you should be doing while driving a car that would require a bigger screen.
For back seat you could get a 14" or 15" for pretty cheap, but at that point why even do touchscreen? If you have that much space to have a huge monitor you have space for mouse and keyboard too. But apparently this computer isn't going to be used by the driver or for things like mp3, gps, etc... so why not just have passengers use a laptop?