The tank size of the urea-based diesel exhaust fluid (DEF) is typically designed to match up with oil change intervals, often only requiring a refill every couple changes.
There lots of good technical detail (probably more than you want) about the VW/Audi/Porsche system here:
http://www.myturbodiesel.com/1000q/audi-q7-tdi-dpf-faq.htm
Start about 1/3 of the way down that page, look for the first section that talks about “AdBlue”, which is the VAG trade name for their DEF.
I suspect I’ll have this on my next truck.
Thanks