I think they're 6.5" on the A6 with 1" tweeters, unless you have the Bose upgrade.
I think only Blaupunkt do interface harnesses for the steering controls, but I maybe wrong. Their HU's are certainly the only ones I've come across that use the bizarre mini-ISO connectors for the pre-outs and CD changer, just like the Audi ones, so it's a straight plug & play from a wiring point of view. I guess this is derived from the fact that they used to make all the head units for Audi's (pre-'99) before Matsushita took over, but still use the same connectors.
Blaupunkt certainly list an adapter cable for the A6 (97-04) for the steering wheel controls, part number 7 607 621 145 for use with most of their HU's and they are one of very few manufacturers who do a decent all in one DAB tuner (Woodstock DAB54), which I currently have fitted in my B6 A4 and can't rate highly enough. It has Aux inputs for iPOD, plays MP3 CD's and can even record off air onto SD media cards. A snip at £299.
Also the A6 uses a seperate amp for the rears and sub, but when I tried linking into it I had to fade quite a bit to rear in order to get a balanced sound. By passing the rear amp and using the rear speaker output from the HU cured this, but the sub was then too faint, so ended up ditching the plastic sub and installing a seperate amp and JL Audio sub in a proper MDF stealth enclosure. But TBH you can get quite a good sound just from the cabin speakers as they are mounted in the doors rather than on the flimsy door cards.
