From the Android developer site:
Size and Format
Launcher icons should be 32-bit PNGs with an alpha channel for transparency. The finished launcher icon dimensions corresponding to a given generalized screen density are shown in the table below.
Table 1. Summary of finished launcher icon dimensions for each generalized screen density.
ldpi (120 dpi)(Low density screen) 36 x 36 px
mdpi (160 dpi)(Medium density screen) 48 x 48 px
hdpi (240 dpi)(High density screen) 72 x 72 px
xhdpi (320 dpi)(Extra-high density screen) 96 x 96 px
You can also include a few pixels of padding in launcher icons to maintain a consistent visual weight with adjacent icons. For example, a 96 x 96 pixel xhdpi launcher icon can contain a 88 x 88 pixel shape with 4 pixels on each side for padding. This padding can also be used to make room for a subtle drop shadow, which can help ensure that launcher icons are legible across on any background color.