On my starting page there's no need for a vertical menu. Only when I select Entries in the top menu I want to see a vertical menu.
Is it possible to stretch the startsite (wheres no side menu) on the whole width of the site? Means using also the empty space from the menu.

yes, you can turn off all modules on right position.

This is what I tried first. All modules "right" are disabled. Could be the reason I moved the menue from right to left with this code:

#jf-main{
float:right;
width:680px;
margin-left:20px;

YEs you are right
change the values width seems be the only possible because unplublishing the right modules don't affect the layout width. (hope I wrong...but it is what i saw)

Ny question is:
Is there a way to change just the width layout for the articles pages?

because changing the #jf-main{
float:left;
width:680px;
margin-right:20px;
it s affects of course all the website..layout

You want turn of right postion on article only ?

I wanted to increase the width just for the article (I unpoublished all the right modules)

I found that if I post a large image more than 640px ( the widht size of the jf main) I obtain what I was looking for..
the article page widht increase.
so perfect. Like this

thanks anyway!