Wallpaper size for iphone 6 is 750 x 1334 pixels
6 plus is: 1080 x 1920 pixels
You can try this app:
Retina HD wallpapers
This one is free and only wallpapers for iphone 6 , 6 plus
Re:
I think macbook air reached limited about thin, light.
The last things is about retina screen and battery life.
One thing i don't like about mb air is the screen. too easy to damage or scratch