GOT IT!
for those who eventually need to know...
if you need to have larger or smaller article width, you must change in the file
jf_relaxio/html/com_content/category/blog.php
in the line
$width = 890*count($this->intro_items);
you can change 890 with 590 for example
and in the file
jf_relaxio/css/template.css
in the line
.items-row{width:865px;float:left;margin-right:25px;}
you can change 865 in 565
please note that width + margin-right of the css it must be the same value you set in the php file.
hope all is clear.