There are many phones, each with it’s own icon size (for example, Nokia has 19 different sizes, http://www.j2meforums.com/wiki/index.php/Application_Icon_sizes).
I would like to use only one icon (even if it’s displayed poorly).
Is there a single icon size and color depth that displays in all phones?
There is no standard size, and some phones will not even show the icon if it is not the exact width and height specified for the device. Some phones may attempt to scale your icon up or down, depending on what it requires. However, you can’t rely on this. Just another fun part of the highly fragmented mobile space 🙂