My brother has the 15 inch which is fine for him because he edits/ DJ's/ is a gamer, so he needs the screen space. I use a 13 inch which is perfect for me because I use if for basic stuff, documents, powerpoints, music, films, internet. It's small enough for me to carry it around without it being too small that I can't see what I'm doing. Unless you're a gamer/ need it for media I wouldn't recommend going as big as 15 inches.
Take your money and burn it. A Mac is a waste of time unless you are utterly PC illiterate or running a server and dive into unix alot.