Icon Size in Android

For  Launcher icons :

48 × 48 (mdpi)
72 × 72 (hdpi)
96 × 96 (xhdpi)

144 × 144 (xxhdpi)
192 × 192 (xxxhdpi)
512 × 512 (Google Play store)

For action bar, dialog and tab icons :

24 × 24 area in 32 × 32 (mdpi)
36 × 36 area in 48 × 48 (hdpi)
48 × 48 area in 64 × 64 (xhdpi)
72 × 72 area in 96 × 96 (xxhdpi)
96 × 96 area in 128 × 128 (xxxhdpi)

For contextual icons :

16 × 16 (mdpi)
24 × 24 (hdpi)
32 × 32 (xhdpi)
48 × 48 (xxhdpi)
64 × 64 (xxxhdpi)