Is a 15 inch laptop enough, or is it enough for a 17 inch laptop?
It depends on you. I'm 15 "too big - I just got one with 12"
15.6 "is the maximum I want to carry around with me, I'd rather have a 14" notebook.
For what should it be enough?
To kill the cat? 15 "are enough.
Personally, I find everything about 13 "too bulky for a laptop.