My neighbor is doing his thermostat/rad/ac and just borrowed the digital caliper and I cant find the other one I have. Too late to wake him up now (1am) but I can measure inj return lines tomorrow afternoon.
Inside diameter might be 3.5mm, not totally sure.
For obvious reasons, you might consider
diesel rated translucent lines
http://www.ebay.com/itm/VW-3-5mm-Die...-/360497816020
http://www.ebay.com/itm/3-5mm-Diesel...0c01c5&vxp=mtr