This patch adds a new theme configuration option, TaskBarMaxHeight, which sets the height of the taskbar in pixels (but makes it narrower only, never higher). This may of course cause some taskbar widgets or text to be truncated. Note that the taskbar is actually one pixel higher than the number you give here, because of the one pixel border.
I need this option because in some builds at least it seems impossible to make the taskbar narrower with the existing configuration options.
The patch is based on icewm-1.3.7 vanilla.