For this kind of job you should install the Firebug extension in Firefox. It will allow you to investigate the CSS easily and even try live changes on your site to see if it works.

Thx, I will try right now, already looked on lots of css files and could not find anything about it.
I saw that 2 bottons uper right that dinamccly change width here on forum, will try to inspect it to find out
thx again

Finnaly found it
was under styles.css on skin directory
.posts and div.wrapper2
and it was using relative unitys (ex) instead pixels, so was hard to find
thx for firebug tip

great success! ;)