It's influenced to a certain extent by the size of laptop that you are willing to live with, which is, in turn, influenced by how and where you will use it.
On a smallish laptop, without going too small, a 15.4" widescreen is nice. A widescreen has three plusses; the chassis will be quite shallow, the keyboard won't be too narrow, and of course, word processing and web browsing are well accomodated.
If you'd prefer something more akin to a desktop replacement, then a 17" screen is desirable.
14" screens a bit too small for much more than occasional use, but the laptop will be small and light.
Remember that screen size/shape dictates chassis size/shape and thus influences weight. It's all about compromise.