Please see my comment to the question.
My idea is: take a good
rounded corner generator. You will find a plenty of them:
http://bit.ly/POC2pp[
^].
I used this one:
http://cssround.com/[
^].
The aspects of browser compatibility are explained here:
http://www.css3.info/preview/rounded-border/[
^].
Don't trust it fully, better check up with browser documentation on each well-known browser.
It will give you the cross-browser code (if a generator is good). Now, choose
width
and
height
exactly twice as radius. If you want a border, slightly change the size, by the size no bigger then border width. You are done.
—SA