Are the rear and front wheels the same on the 2005 Audi A6 Quattro

For tire size or rim size, you can crosscheck on your tires.

All tires have the size and spec stamped on the side. Which you can use it for reading the size of the tire and rim. So whenever you want to change the tire just look at your old tire and match it. (Unless the old tire is somehow using the wrong size?)