How do we get the proper USB charger for tablets with varying cable lengths and gauges?
Consider "power-over-ethernet-to-usb" e.g http://www.ipadenclosures.com/ipad_kiosk_enclosure/accessories/poe-to-usb-adapter-for-ipad

That uses 48V along the Cat5 cable, and a converter at the recieving end. So power dissipated in the cable is minimised and you're guaranteed a nice stable 5V over the charger.