You must define dpi on your emulator. 240 is hdpi, 160 is mdpi and below that are usually ldpi.
Extract from Android Developer Guide link above:
320dp: a typical
//actually you get confused
here you mentions 320,480,600 and 720 is width of the device in portrait mode.
//this image show how to create 320dp screen as 240 as hdpi or you can make it for 320 xhdpi emulator
Note: here 213dpi is new tvdpi for nexus 7 devices.