There are a few major of density values for our Android devices. And They are: ldpi (about 120 dpi), mdpi (160 dpi), hdpi (240 dpi), and xhdpi (320 dpi). But, how did it ended up in these values? Here’s a simple explanation by Dianne Hackborn, the Android Framework Engineer.
Speaking of screen sizes, you may need to read this information about multiple screen support written by Android developers.
And more, tips designing for Android by Smashing Magazine.