those are 2 different questions:
The first one doesn't make sense, because the size of the 15" macbook pro didn't actually change (for some years now), so no, it didn't "become too big".
Is the 13 the new mainstream? Maybe, since it has both a MBP and a MBA, but the 11 is mainstream too. But does that matter, or does your preference matter?
I used to like big screens, watching movies on them etc, but now I have a 11" macbook air. It's so small and light you can carry it anywhere without a problem. Some people can carry a 15" without a problem too, but I wouldn't...
Plus the high resolution of the macbook airs make up for the small screen, you can enjoy HD movies on it .
what is the new mainstream size, I think it was 15 and is now 13