User experience is an art and it is possible only when patterns and design guidelines are followed. Below are the systematic approaches to Android design specifications.

Launcher Icon

Size(px) Screen Density

48 × 48 mdpi
72 × 72 hdpi
96 × 96 xhdpi
144 × 144 xxhdpi
192 × 192 xxxhdpi

These are the icons that are used as the application logo in the device launcher.

Best Tools:

http://romannurik.github.io/AndroidAssetStudio/icons-launcher.html

http://makeappicon.com/

Action bar, Dialog & Tab icons

Size(px)(24 dp Icons) Screen Density

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

These are the icons used in action bars.

Best Tools:

http://romannurik.github.io/AndroidAssetStudio/icons-actionbar.html#source.space.trim=1&source.space.pad=0&name=ic_action_example&theme=light&color=33b5e5%2C60

Small Contextual Icons

Size(px) Screen Density

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

Small contextual icons are used in most of cases to describe the course of action, For example: Like, Comment, and Share icons on Facebook wall post screens.

Best Tools:

https://iconverticons.com/

Notification icons

Size(px) Screen Density

22 × 22 area in 24 × 24 mdpi
33 × 33 area in 36 × 36 hdpi
44 × 44 area in 48 × 48 xhdpi
66 × 66 area in 72 × 72 xxhdpi
88 × 88 area in 96 × 96 xxxhdpi

The notification icons are used to represent application notifications in the status bar. These icons are always should be clear, flat and non-vector icons.

RatingBar Star Icons:

Small Star

mdpi: 16px
hdpi: 24px
xhdpi: 32px
xxhdpi: 48px

Medium Star

mdpi: 24px
hdpi: 36px
xhdpi: 48px
xxhdpi: 72px

Large Star

mdpi: 35px
hdpi: 52px
xhdpi: 69px
xxhdpi: 105px

Best Tools:

http://romannurik.github.io/AndroidAssetStudio/icons-notification.html#source.space.trim=1&source.space.pad=0&name=ic_stat_example

http://resizeappicon.com/

For further info please refer developer site.

Splash background sizes

Size(px) Screen Density

320x480 drawable-mdpi
480x800 drawable-hdpi
720x1280 drawable-xhdpi
1080x1920 drawable-xxhdpi
1440X2560 drawable-xxxhdpi