I used to develop for Android and iOS, and for multiple screen support we use to use mdpi, hdpi, xhdpi.
Now I want to develop for Windows Phone, I want to set a background to a button and I don't know how to define image size to support all screen sizes.
You dont need to provide different assets. See info from MSDN below. There is a way to support different assets/resources. But we have never done this bevore in our apps.
From MSDN:
More info: Multi-resolution apps for Windows Phone 8