I suppose there are some people like me, had puzzled with rich:selec's width setting for long time.
I just solved it and share the way with you all.
We don't need to create a class of css, let's just override the existing class of rich:select in a new css file named select.css in resources folder.
Then, let's add this css file to the page on the top of body inner and one line jquery script to change the css style.
<h:outputStylesheet name="select.css" library="css" />
Run the page, please. Yes, we changed it!