I had the very same question while while starting out. Do take a look at the detailed answer I later posted. [How does Unity size graphics][1]

> When we set camera size, there's a certain screen height. This height
> is fixed irrespective of the device screen. On larger devices, greater
> detail, on smaller devices, lesser detail. So, if I chose 760px as
> height of my background image, I calculate camera size as follows:
> cameraSize= (Height/2)/PixelsPerUnit which in my case is,
> (760/2)/100=3.8
> 
> Next, I need to make sure skybox doesn't show irrespective of the
> device's screen size. This means, design background for the widest
> aspect ratio which is 16:9. So, I calculated the width as
> 
> width=AspectRatio X height. Which in my case is (16/9) X 760=1352 px.
> 
> So my background is 1352 X 760 pixel. This fits 16:9 aspect ratio
> perfectly. On narrower ratios like 4:3, there is cropping towards the
> sides. But skybox never shows. Which was my main issue.

 [1]: https://gamedev.stackexchange.com/questions/95946/how-does-unity-size-graphics