Of course the size depends on the usage. If you need a notebook to take to classes and you just write papers, notes and surf the net then something under 15" would be fine. For the regular traveller who does other stuff as well with his/her notebook, 15" (or 15,4") is great. For those who need desktop replacements I think 17" is pretty great.
Actually I just bought a 17" DTR-notebook and I was really afraid that this thing would be too big. However, the size was just perfect (I have FSC M3438G which is pretty thin notebook as opposed to Alienware which is pretty bulky). 19" notebook would be really pushing it because with this 17" notebook the screen just right because if it was bigger I'd miss a whole lot of information when gaming or something.
So my answer is that unless you want to look your notebook's screen from a distance or you are going to use the notebook as a 'hometheatre' then 19" is way too big. I can carry my notebook quite easily though it's not that great in my lap. Carrying a 19" notebook would be quite difficult and honestly, you'd always have to find big enough desk to put in on although you probably would keep it in the same place all the time.