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.
- Change it in your css file (would affect the whole site)
- Create a custom page template and change that by adding
Some css onPage..
- 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.