lukezero
4:3 Aspect Ratio
320x240 Android devices
1024x768 iPad 1, iPad 2
2048x1536 iPad 3
3:2 Aspect Ratio
480x320 iPhone 3GS and lower, Android devices
960x640 iPhone 4, iPhone 4S
16:10 Aspect Ratio
800x480 Android devices, WindowsPhone7
1280x800 Android tablets like Google Nexus 7, Samsung Galaxy Tab 10.1, Motorola Xoom, Asus Eee Pad Transformer
17:10 Aspect Ratio
1024x600 Android tablets like Samsung Galaxy Tab 7
16:9 Aspect Ratio
640x360 Symbian3 devices like Nokia C7
854x480 Android devices, MeeGo N9
1136x640 iPhone 5
But if you want to be safe for mobile phones: just choose either
Landscape:
1280x720 pixels for high quality
or
854x480 pixels for medium quality
Portrait:
720x1280 pixels for high quality
or
480x854 pixels for medium quality