The following are the best dimensions for the app to run in all devices. For understanding multiple supporting screens you have to read
http://developer.android.com/guide/practices/screens_support.html
xxxhdpi: 1280x1920 px
xxhdpi: 960x1600 px
xhdpi: 640x960 px
hdpi: 480x800 px
mdpi: 320x480 px
ldpi: 240x320 px