I do believe 13" notebook are gaining the fastest in popularity compared to 15 and 17" notebooks.
The standard used to be and to a large degree still is "get a 15" notebook, it's the best all around computer"... That's changing I think, the fact that (nice) external displays are now much more affordable and that people more and more like using and taking their notebooks "everywhere" plus a smaller (13") notebook is much more comfortable to have on your laptop, 'grab and use', even just around the house it's much less cumbersome than 15" notebooks, yet you're getting a full size keyboard and good screen.
I've owned a 15" MBP and for me the little bit you gain in screen real estate isn't worth the extra weight on your lap or the extra space it takes up on your desk, plus the weight in your bag ... 13" = more enjoyable laptop experience, imo...
13" is going to outpace 15" very soon, even sooner for Apple with the new 13" MBP...
Anyone else agree/disagree?