You should be more than fine with a 16-25' AS & probably more, as Andy T at CanAm reviewed the 2013 Cay-TDI with a 27 or 30' - see the hitch hints article at the link below, then call Andy at CanAm for more info...
Can-Am RV :: Hitch Hints
..... see HH 41-7
If not already a member, join PCA & get on some of the Cayenne owners forums online - Pelican Parts has a good one, & a few others are out there; then look for topics from folks who actually do tow with theirs.
Also ditto for the cousin Taureg online forums, where their V10 & V6 TDIs have been in service for a longer time (essentially the same 3.0L V6-TDI as in your Cay, but without the "Porsche magic tweaks" added to the drivetrain & suspension).
Here's another topic on V6 vs V8 Cayennes, where some TDI owners have weighed in ....
We're currently looking for a 2006-10 Cayenne S to tow our vintage Avion T20 - which is probably only 3000# wet & loaded - & are focused now on the Iceland Silver 2006 Cayenne S Titanium Ed. because we like the color & it may be
a Cayenne model/trim which eventually holds then improves in value over time, since I tend to hold onto my vehicles long term.
You can read some CayTDI input at my topic here...
We can't justify the extra $20-30k for the later diesel for a few mpg better on more expensive than premium fuel, on what is essentially an "extra car" that we also use tow to vintage trailer events & my 73 914-2.0 to PCA concours, etc. So we're fine with the V8 gas pre-owned option for the 3-5000 mi/yr we'll use it.
As with 2012FB's comment on his wife's use of their MBZ, my wife won't drive big trucks/suv's, & they won't fit in our driveway anyway - so they never were an option.
FYI - our set-up is a Hensley Cub WD/Anti-sway hitch (smaller than their Arrow for up to 6000# trailers), with a Tekonsha Prodigy RF trailer mounted wireless brake controller (it's plug-n-play for rented TVs until we get our CayS).
That one is the same as their hardwired P3 under-dash box mentioned above by 2012FB, which you could get (you'll need the wiring pigtail to hook it up), unless you too anticipate having more than one TV to share duties - then go with the wireless - or if you prefer leaving the Cay unmolested & just plugging the wireless hand controller into the 12v
outlet in the Cay, with the controller box mounted on your trailer's A-frame (see pix at my link above of ours).
Enjoy your new Pepper - both hitched & "free"!!
And don't forget to entertain your PCA friends by bringing the shiny new AS hooked-up to the Cayenne to some events ... just to show off what they cannot do with their 911s, Boxsters & Caymans!