It really depends on what the person in question is after...
Someone who prioritises portability will most likely be going for the 13", unless they have any other specific reason for wanting the 15" (screen estate, but then, a person who wants the most portable is probably not so fussed about screen estate and is more concerned about it being that much easier to carry around all the time).
Someone who wants a desktop replacement will likely go for the 17" and be carrying it around less often.
Someone who wants a bit of both (and the price to match) will go for the 15"!
I've had 12", 13", 15" and a 17" laptop of varying makes over the years. I took the 17" with me on planes and stuff but it was a song and dance, and the charger was damn heavy too. The 12" was much better and meant I could carry more in my bag. My 13" isn't much different, and has the bonus of being thinner.
The 15" I never got to carry around that much, though.
I pick 12/13" because there are times when I carry it for about an hour every day, so it is a big deal to me. Plus it's nicer to watch things, or work on a plane. However, I also have a main desktop PC (which is now an iMac). If I didn't, I might consider a bigger size...
I probably sound like a spoilt computer brat, but the sad fact is I'm so busy with work I don't have time to spend money socially, so it all goes on computer toys

D)