Hey Roberto,

Are you talking about the overall width of the forum? If so, this is controlled by the WordPress theme itself not the forum theme, as the forum sits in a WP Page. Changing the width should be pretty straight forward through the WP theme style.css, but of course this is different in some themes!

Thanks…Yes I talking width forum…….now I see in my Css file Twenty thirteen theme,  But it will also change the layout of my normal pages? you know the precise point of css file? thank you

It looks like you would have to change:

.entry-header, 
.entry-content, 
.entry-summary, 
.entry-meta {
    margin: 0px auto;
    max-width: 604px;
    width: 100%;
}

As you can see it currently has max-width: 604px which is limiting it. You can change that to whatever value you like of course, it will change every page.

I think you can create a custom class for a page and items using body_class so you can have a different width for the forum without affecting other pages, but I’ve never used it so I have given you the link to the Codex entry to read at your leisure.

I think there are also a couple of WP plugins that will allow you to easily add a custom class to a page (probably intended for multisite installs using the same theme throughout)

Very very thank I solved 

No problem, glad to help