How to use the full page

In the theme you have choosen (twenty ten) – the main Container
where you page / post is posted is the div:

<div id="content" role="main">
<!-- YOUR CONTENT IS PUBLISHED HERE -->
</div>

.
This div has a maximum width of 640 pixels…
if you want to change that you can do one of three things.

1. Change it in your css file (would affect the whole site)
2. Create a custom page template and change that by adding
   Some css onPage..
3. use custom fields to embed the css in to your header
   (my favorite way but need some coding <– a little bit)

.
Hope this helps… its just a css issue.. not really a wordpress one.

Cheers, Sagive.