The best example I have seen is foursquare's open source android version. If you download the source you'll be able to browse through the res/drawable-xxx
folders and see how they handle various cases with their icons that appear to be approximately the same size as, say, facebook's android icons (maybe a pixel or two more padding).
I recommend checking out this project and simply examining their files in your photo editor of choice.
Just as a note: foursquare is no longer open sourcing their app (and hasn't for some time now...), so the code found at the above repo may be a little stale.