the only computer i own is a 13 in macbook pro
works out great for me. perfect size to put in your backpack and take to class. i do everything on it (including playing games like cod4) and havent found much problem.
ultimately i think 15 in would probably be the perfect size. small enough to carry around and big enough screen estate for gaming and movies.
While I agree in a lot of ways the 15" MBP can be the 'perfect size' and I try and use my 15" as often as I can for me it just hasn't turned out to be a good 'take everywhere / use anywhere' notebook.
My computer is used at least 10 hours daily for business and personal tasks, from entertainment, heavy work, video's, forums, 20 tabs open at a time kind of usage.
I've thought the bigger the screen the better, that's true in many cases, I have a 65" TV and love it, my 24" ACD is really nice, although I rarely use it.
I'm a CEO and can get 20 to 100 emails a day, plus I always like to checking the pulse of the business throughout the day. Our offices are 95% Mac's now, we have anything from MBA's, MBP's iMac's and MP's... I can use any computer I like, including a Sony Z, Lenovo x301, a few Dell's plus blackberries iPhones etc.
As it turns out for me the 13" MBP is currently the best notebook on the market for my needs. While the display on the 15" is beautiful it is too bulky for a computer I'll take from my desk to my lap on the couch, deck, kitchen, car, cottage, meetings etc. On the other hand the 13" is real easy to grab and go you can use a smaller backpack / messenger bag too. 7 hour battery life, sd card slot, 256ssd, 8gb ram my 13" is very fast, screen is perfect I really like it. Wish it was lighter, thinner and narrower without the wide bezel, and of course an AG option, no perfect notebook yet.
The Air, it's a great computer, would be my choice if the battery lasted longer, had more ram and ran cooler.