The 15" is the sweet spot between a portable 13" and a not as portable 17". But duh, you knew this already, as 15 is in between 13 and 17. What you really meant to ask was, will a 13" screen feel too cramped or will a 17" be too large to be portable? Will a 15" be both portable, and not cramped?
Well, the simple answer is not at all straight forward. The 13" screen is more than large enough for most work - non photoshop or final cut - and as it's the most portable, at first glance sounds like the right choice.
However, just because you have 13" to work with doesn't mean you have 13". On a PC, we are used to working with maximized windows. on OS X, as you may (or may not) know, the norm is to have a window that is not fully maximized, and only fills about 85% of the screen. Therefore, on OS X we can get about 13" of usable screen real-estate on the 15" MBP, and less than 12" on the 13" MBP.
I have a 15" MBP, and it's the right size. Any smaller and there wouldn't be enough screen real estate, and any larger would be too large to be easily portable. But don't think it's because 13" is too small - if you really want to use OS X in a way that it was not designed to be used, and work with the windows maximized, a 13" MBP will be plenty large enough for 99.9% of the things you do.