That's a massive range of screen size and operating systems to consider (just from android phones!).
Have a look at this simple graphic showing the different screen sizes:
The problem is that developing apps to work across this full range can be challenging, on the other hand android has a bigger global reach than iOS and potentially a lot more customers.
There is no way you can test on all eleven thousand varieties!! Is it time for HTML5 web standards as the way forward?
You can download a PDF of the report here..