There are precise guidelines published in reference to Android icons. Different design guidelines are published for tab icons and menu images.
Tab icons represent individual tabs in tabbed UI. Tab icons should be provided in two distinct states: unselected and selected. It is required that tab icons are drawn as simple, flat shapes as opposed to images shown in 3D or isometric projection.
Android OS and Android apps can run on a variety of different platforms employing a wide range of hardware. In reference to icon graphics, those devices can have different screen sizes, display resolutions, pixel dimensions and density. To accommodate the wide range of displays, Android developers must supply all tab icons used in their apps in at least three resolutions to be drawn on low, medium, and high density screens. Resolution-wise, the outer boundaries for the three resolutions are defined as 24×24, 32×32, and 48×48 pixels. The size of the shape itself should not exceed 22×22, 28×28, and 42×42 pixels respectively. By supplying all three standard sizes, developers can ensure that their apps will display properly on a wide range of devices running the Android OS.
Menu icons are drawn in the “options” menu, and are shown to users when they press the Menu button. Similar to tab icons, menu icons should be flat, grayscale images. Just as tab icons, developers should not use 3D or isometric projections.
Screen sizes for menu icons are described in a slightly more comprehensive way compared to tab icons. Instead of just two sizes described for tab graphics (inner shape and boundary box), the inner shape of menu icons can be smaller or bigger depending on their shape. If a menu icon is square-shaped, its resolution should be smaller than for icons shaped otherwise. The reason for having two different sizes is to establish a consistent visual weight across the two icon types.
The outer dimensions for low, medium, and high resolution icons are defined as 36×36, 48×48, and 72×72 pixels respectively. Inner shapes for square, low-definition icons should be 22×22 pixels, while non-square icons should fit into a boundary box sized 24×24 pixels. Similarly, mdpi icons should fit 30×30 and 32×32 pixel boxes, while high-definition images should fit into 44×44 and 48×48 pixel boundaries respectively.
Instead of designing your own graphics conforming to these guidelines, Android developers have an option of getting ready-made images from professional designers. As an example, Android Tab Icons by Aha-Soft include 112 unique tab icons in selected and unselected states and all three standard resolutions. Should additional resolutions be required, developers can render graphics at any resolution by opening scalable vector sources. Android Tab Icons can be viewed and downloaded at www.aha-soft.com.