Here is a very generalized handy view, but an app should support this at the very minimum:
Android
- Standard (HDPI) – 800×480 (WVGA) (phones: HTC Desire, Galaxy SII)
- High Res (XHDPI) – 720×1280 (720p) (phones: Galaxy SIII, 7inch tablets ~approx)
- Optional: Tablets (XHDPI) – 800×1280 (WXGA) (10 inch tablets, e.g: Transformer Prime)
iOS
- IPhone 4S – 640×960 pixels, 3.5 inches (~330 ppi)
- IPhone 5 -1136×960 pixels, 4.0 inches
- IPad – 2048×1536 (264 ppi)