As my only computer, I would want a 15". 17" is too big, and the screen on the 13" is just too small to use all day. As a secondary computer (which it will have to be if Apple keeps removing features), I would want a 13" since it is more portable, and I would use my primary computer (a Desktop with a large, 22-24", LCD) to get work done.
When I bought my 15" MBP, I almost bought a 13" MB, which would have been about $700 cheaper and just as fast except for the video card. Then I would have put the $700 into upgrades and an LCD for my desktop. Then I would have used the Desktop for computationally intensive work like Matlab (since I would have put in a fast CPU), video editing (because the Desktop can access large disks quickly while an eSATA lacking macbook can't), and gaming (because the desktop would have a real video card).
I ended up getting the 15" and used it as my only computer all last year in college. The screen size was never too small, and it is portable enough. It couldn't do everything, though, since virtual Windows is too slow, and I had to reboot to play any games, so this year I am going to upgrade my desktop on the cheap and have the best of both worlds, except for portability.