Use whole page from left to right

is that in the dashboard when editing/writing static pages or on the public side of the site?

you might need to edit the html structure of the template; or adjust the css for the page.

please post more details:

– name and download link of your theme;
– link to your site.

for hte editor width, Twenty Ten uses editor-style.css;

try to change it there:

html .mceContentBody {
	max-width: 640px;
}

in style.css, for pages without sidebar, this style is restricting the page content width:

/*
LAYOUT: One column, no sidebar
DESCRIPTION: One centered column with no sidebar
*/

.one-column #content {
	margin: 0 auto;
	width: 640px;
}

where would need to change this

general, for all single column-pages, you would find the suggested code (which was a straight copy from style.css of Twenty Ten) and change it there (not too far from the top of style.css).

Can I do this for individual pages?

yes – add the ‘body_class’ output for that page before the code;
http://codex.wordpress.org/Function_Reference/body_class

one of the body_classes for a static page is .page-id-123 where the number is the page ID.

example (for your test page):

.page-id-471 .one-column #content { width: 940px; }

add this either at the end of style.css;
or use a custom css plugin for any edited styles – for instance the custom css section of the ‘jetpack’ plugin.