There are strict guidelines published in reference to Android icons. Different design guidelines are published for tab icons and menu icons.
Tab icons are drawn in individual tabs in tabbed interface. Tab icons must be supplied in two distinct states: unselected and selected. It is required that tab icons are drawn as simple, flat shapes as opposed to graphics shown in 3D or isometric projection.
Android OS and Android apps are used on a variety of hardware platforms using a wide range of hardware. In connection to icon images, those devices can have different display sizes, screen resolutions, pixel dimensions and density. To accommodate the wide range of displays, Android developers must provide all tab icons used in their apps in at least three resolutions to be drawn on low, medium, and high density displays. Pixel-wise, the outer boundaries for the three sizes 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 providing all three standard resolutions, designers can ensure that their apps will display properly on a variety of platforms running the Android OS.
Menu icons are drawn in the “options” menu, and are shown to the user when they press the Menu button. Similar to tab icons, menu icons should be flat, greyscale images. Just as tab icons, developers cannot use 3D or isometric projections.
Screen sizes for menu icons are described in a slightly more comprehensive way as opposed to to tab icons. Instead of two sizes defined for tab icons (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 specifying two different sizes is to establish a consistent visual size across the two icon types.
The outer dimensions for ldpi, mdpi and hdpi icons are defined as 36×36, 48×48, and 72×72 pixels respectively. Inner shapes for square, low-definition icons is 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 boundaries, while high-definition images should fit into 44×44 and 48×48 pixel boundaries respectively.
Instead of designing your own images matching these guidelines, Android developers have an option of ordering stock graphics from professional designers. As an example, Android Tab Icons by Aha-Soft include 112 unique tab images in both states and all standard sizes. Should additional resolutions be required, developers can render icons at any resolution by accessing scalable vector sources. Android Tab Icons can be previewed and downloaded at www.aha-soft.com.