iOS:
For iOS, we have to provide 2  resolution graphic assets, one for old screens and one for new retina screens
Old Screen Resolution : 320X480, 320×568.
New Retina Screen Resolutions : 640×960, 640×1136.

Ipad
Old Screen Resolution: 768×1024
New Retina Screen Resolutions : 1536×2048

Android :
Android application has numerous resolutions. To make simple for designers it has categorized into four general sizes
which are  ldpi (low), mdpi (medium), hdpi (high), and xhdpi (extra high).

To avoid worrying about changes in screen density.

• xlarge screens are at least 960dp x 720dp
• large screens are at least 640dp x 480dp
• normal screens are at least 470dp x 320dp
• small screens are at least 426dp x 320dp

View-Port is also  a good solution for responsive web design which you can target divices. Possible view port sizes are listed in the below link.
http://i-skool.co.uk/mobile-development/web-design-for-mobiles-and-tablets-viewport-sizes/