I don't have a 6800GS card, but I do have a long history of NVidia cards, including the 5600U, 5900, 6600GT and 6800GT... family of cards tend to all be the same length... a 6800GT tends to be the same length as a 6800NU and a 6800U, but longer then a 6600GT.
I can't remember exact lengths, but I do remember how long they are in order of shortest to longest:
Shortest
6600GT
5600U
6800GT
5900
Longest
Both the 5900 and 6800GT were too long to fit in one of my smaller HTPC cases, while the 6600GT and 5600U both fit into that same HTPC case. I needed a fairly roomy case for my 5900 and 6800GT.
While I don't have a GS, I would imagine it's roughly the same length as my 6800GT. I can't remember the exact length, but if memory serves me correctly, I think the 6800GT was about an inch or inch and a half longer then the 6600GT (I had to get a new case before I could use the 6800GT myself when I upgraded from a 6600GT).
Oh, and I love the performance differance of the 6800GT over the 6600GT... if the GS can unlock to a full 16 pipes, it's worth the upgrade.