Major device sizes

small: mostly ignored
normal-mdpi: older phones with about 3 inch screens, 480x320px resolution, e.g. MyTouch
normal-hdpi: most phones with 3 to 4.3 inch phones with 480 wide screens: Droids, Nexus, etc.
large: 5 and 7 inch tablets: Samsung Galaxy Tab 7
xlarge: 8 to 10 inch tablets, eg. Motorola Xoom, Samsung Galaxy 8.9 and 10.1

Uki's Google Java Technology blog