Re discs, as Gup says the rears are all but impossible without quite a lot of expenditure on special tools because of limites access due to the quattro system. With the tools it's a doddle. The quote from my local VAG indie specialist was only £30 more thanparts from Eurocarparts, so I let them do it all. You'l know if the discs need doing by checking how Jagger-esque the lips around the outer edge are.

As to sizes - they're easy enough to measure with the wheels off.