The 2013 Audi A6, like its BMW 5-Series, Jaguar XF, and Mercedes-Benz E Class luxury rivals, has less space inside than you might think from the outside. Technically it's a five-passenger sedan, but with a smallish trunk and a a relatively skimpy middle (fifth) seat space in back, you'd likely be happier with the (arguably more stylish) four-passenger A7 hatchback if practicality is a top concern.
With an overall length of about 193.9 inches, the A6 is about the exterior size of many mid-size sedans, although Audi has made an effort to give this sedan the swept-back proportions of a rear-wheel-drive sport sedan, and that's cost some space. With about two more inches of height versus the A7 hatchback, there's a touch more headroom in the A6, though.
Front-seat accommodations in the A6 are superb, with lots of head, leg and knee room, though the A6's center console widens a bit and nibbles out a bit of the available space. We've found the power front seats to be supportive and exceptionally comfortable on long trips, and Audi recesses its headrests for far better comfort than the protruding active headrests on some other luxury cars. Front seats are heated as well, and on the most expensive models, they're ventilated for hot-weather comfort.
Functionally, the A6 has some key differences versus the A7 in back-seat accommodations. Two six-footers can fit back there, although knees may be mashed against the front seat. On the other hand, the A7 feels a bit roomier, though both entry and headroom are tighter due to the lower roofline.
Trunk space is obviously smaller than in the A7 hatchback, but at 14.1 cubic feet it's even a bit small by luxury-sedan standards; the cargo floor is also quite high. All A6 and S6 sedans have fold-down rear seats, although they're not completely flat-folding.
And seemingly in following the tradition of German luxury sedans, there isn't all that much storage space for smaller items. Cupholders are small and maintain a tight grip--which might only be to your liking if you drink mini beverages. Doors do contain carved-out areas for water bottles, but the armrest bin is shallow, and the glovebox runs lean.
Overall, the A6 family of sedans has excellent fit and finish, a tight cabin feel, and an interior that blocks out road noise a bit better than the smaller A4.