I need to reduce the width of the following buttons (drop-downs):
How can I do it?
You can modify the CSS of the underlying button element.
If you want to target specific tools, just use firebug to dig into the DOM and see what their particular classes are.
For example, to modify the widths of the format and font drop downs, use the following CSS:
.cke_skin_kama .cke_format .cke_text,
.cke_skin_kama .cke_font .cke_text {
width: 72px;
}