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?
Depending on the number of handsets you need to support, it may not be realistic to do so. To address this, you can have unique builds created for each handset. Not an optimal solution but if you are trying to port an app to 20 + handsets then icons will be the least of your issues.
I would suggest trying the smallest of these 19 sizes. If you really don't care about how poorly the icon is displayed, a "lowest common denominator" approach could give interesting results.
SVG icon support is in its beginnings with Series60 5th edition phones like the Nokia 5800. Maybe there is hope on that front in the long term.
Of course, I completely agree with omermuhammed: Any moderately complex application that needs to support 20+ handsets is going to require several versions of the jad and jar files, even if yo umanage to use the same icon for all of the handsets.
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 :)