Size matters - I always had small ones, but my girlfriend prefers big ones...
First, I've had a 13" plastic MacBook. That was fine, it was the best compromise between portability and usability. So I sticked with the size when I upgraded to unibody. I went through a number of unibodies, all 13" ones. Then I've had two Airs, then the second generation unibody came out, I'm on my second one. Altogether, it's ten, I think. I use a 22" external monitor, nice and matte. I travel a lot and I have the MB Pro on my lap all the time. For me, 13" seems to be ideal.
HOWEVER, my girlfriend doesn't travel as much and even when she does, she tends not to take her 15" unibody MacBook Pro. She wouldn't want a small screen. She liked the lightness of the Airs, but she thought that having such a small screen was a compromise.
We are all different. But I have to say something. If I could, I would definitely get a 17" MacBook Pro with matte screen and for traveling, I would either have an Air or a tablet. After a while, even the 13" MacBook Pro becomes a burden. So, if I really need processing power, than I would just take the big one, otherwise the REALLY portable one.
One more thing... It's so weird that with most manufacturers, the 15" screens are cheaper than the 13" ones. In 2006, for example, getting a plastic MacBook was not much more expensive than getting a comparable Windows laptop. With 15" ones, Apple always had this excessive premium. With the unibody, they extended it to the 13" range. I mean, what is the reason behind the dated processors selling for over £1000? It's just mad.