I have been testing ion mobile for sometime.As per my view a mobile Web application is viewable by users around the world, even if you’re initially targeting only users in a single country or on a single network. Therefore you need to check your application on as many as possible devices, this will cover the risks of application failures and other issues and improve the market value of the app.
For iOS we can check it on all devices as they are few in numbers, but for android this becomes a problem as we have quite a large no. of devices for android.
In my opinion you must test it with the all standard sizes and resolutions, that would make the application secure to a good extent.Also check it with some standard configuration as specified by you for your application.