For years to make re-installation of brake pipes idiot proof, VAG ABS units tend to have different sized threads for left and right - mix of M10 / M12
I would try a scrap yard to source some OEM ones, that tend to be untouched in the all the life of the car.
Or if it has to be new give Bigg Red/ Hel a call.
Is that the pipe ID or OD you mechanic mentioned?
VW moved over from steel brake pipes with the Mk4, where they changed to aluminium, it has a bit bigger diameter than steel.
-ID is the most important, I would have thought plain old 3/16 would be fine.
Is this mod for the improved traction control? - please report back your findings