It's just a transmission cooler, it's not under that much pressure. The only reason they use these fancy hoses for that is for routing.
It's 12mm line. I just measured my spare lines for someone else. Came in at 11.98mm on the digital caliper.
I had a leak where the flex line joins the hard line, which is annoying to repair. But if I had a leak in the middle of a hard line, I would just cut it there and slip on some transmission fluid tubing, and secure it with double fuel injection clamps (four clamps.)
Or you can buy my spare lines. The install is not that awful.
The QuattroWorld website is not affiliated with or endorsed by Audi AG or
Audi of America, Inc. AudiTM, QuattroTM, Avant,
Tiptronic, FrontTrak, A2, A3, A4, A6, A8, S3, S4, S6, S8, TT, TT-S, R8,
RS4, RS 6, Q7, Q5 and allroad quattro are trademarks of Audi AG.