How big is the sweet spot for the vandersteen quatro wood CT speakers?

If you've put the Quattro's deep into the short side corners (along the 13 foot dimension), try moving them out of those corners, a few feet from sides and at least a foot from the back wall. "Sweet spot" is fairly wide, laterally, with Vandersteens, but in my experience sound is a lot more focused when you're sitting down (not standing) in the "zone." The Vandersteen rear-firing tweeter (if turned on) helps ameliorate this problem somewhat. Good luck, and don't be afraid to experiment with speaker and listener position.  

I own a pair of Quatro Ct's myself.  Very enjoyable speakers!
Will your dealer be doing the setup?  If so they should have the tools necessary to dial in the speaker.  I think the sweet spot of the Quatro is less sensitive to lateral positioning than it is to vertical.  Also, don't be too prescriptive in the placement of the speakers and/or listening chairs.  The optimal listening position (sweetspot) is usually a function of all of the conditions that you seem to have pre-determined.  Be flexible...

Thanks for all your input and suggestions. John at audio connection in Verona New Jersey will be doing the install. From what I've read on the internet there shouldn't be any problem, he's very qualified

+1 for Johnny R.
I have no doubt you will have it set up properly.

For me, with my challenging room. I find the 'sweet spot' fairly large and 2 people can listen easily. FWIW, I like to move around and am often out of the 'sweet spot'. I like it just as much, which is why I love Vandersteen so much. It always sounds good.
B

+2 for Johnny R
Agree w gdnrbob, my Vandy 2Cs have an amazing sweet spot