The general rule of thumb is; to get the largest tablet you can afford, [or larger].
But working with a laptop changes that a bit. I would love a 21" Cintiq but I prefer to work with a 15" Mbpro on my drawing table, so I bought the 12" Cintiq WX. It is ‘‘portable’’ whereas the larger tablets are not. I had a little 4x5’’ tablet but gave up on it as it is too frustrating when moving about in large monitors or a MBPro with a second large monitor attached.
I have a 12x18’’ intuos on my workstation but I’ve grown tired of being ‘‘tethered’’ to a big box. I prefer the mobility now that the mbpro’s have the horsepower to handle 3D.
The Cintiq 12 was the perfect compromise for me. I can set up anywhere
(within extension cord reach of an outlet). And of course it is also a 2nd monitor I can hold in my lap with the Mbpro beside me.
I would have to agree with Jbuck. I would not get anything smaller than the 6x11. You won’t regret it. 