https://bestfirms.com/what-is-my-screen-resolution/
640 x 360
If you want to be safe for mobile phones: just choose either
Landscape:
1280×720 pixels for high quality
or
854×480 pixels for medium quality
Portrait:
720×1280 pixels for high quality
or
480×854 pixels for medium quality
****************
What all the other sizes are good for:
4:3 Aspect Ratio
320×240 Android devices
1024×768 iPad 1, iPad 2
2048×1536 iPad 3
3:2 Aspect Ratio
480×320 iPhone 3GS and lower, Android devices
960×640 iPhone 4, iPhone 4S
16:10 Aspect Ratio
800×480 Android devices, WindowsPhone7
1280×800 Android tablets like Google Nexus 7, Samsung Galaxy Tab 10.1, Motorola Xoom, Asus Eee Pad Transformer
17:10 Aspect Ratio
1024×600 Android tablets like Samsung Galaxy Tab 7
16:9 Aspect Ratio
640×360 Symbian3 devices like Nokia C7
854×480 Android devices, MeeGo N9
1136×640 iPhone 5