I regularly use a Thinkpad W500 with a 15.4" screen at 1920 x 1200 resolution for Rhino, Photoshop etc. I think resolution is at least as important as size - this is similar to the resolution of my 24" monitor, By sitting closer to the laptop screen the image can subtend the same angle as the larger monitor, so the apparent image size can be the same.
So with the new generation of smaller machines coming with massively higher resolutions (e.g. the Yoga Pro has a 13.3" screen at 3200 x 1800 resolution), the question might be from how uncoolly close you can bear to peer at the screen!