Expanding the HTML view
First of all, you should't use a Drupal default theme. As when you update Drupal core, you're going to overwrite all your theme files. So if you want to use a default Drupal theme you need to create a sub theme, if you haven't already. Follow this tutorial Creating A Drupal 7 Sub Theme. 

Secondly, it looks small because you have items in your left column. 

Thus, you need to remove the left column blocks. Go to /admin/structure/block and disable those blocks. OR Edit those blocks and in the Show block on specific pages you can control on which pages you want them to show up or be excluded from.

If that still doesn't do it, you need to edit your theme files. Go to /sites/all/themes/YOURTHEME/ folder and edit the page.tpl.php file and get rid off the left column.  

Even after doing this, if you still want it bigger, then right mouse click on the page and select inspect element. Find the name of the div class or id that is controlling your output. 

example &lt;div id="hello" class="cool"&gt;

then on your css you put 

#hello { width: 5000px; } if its and ID

.cool { width: 5000px; } if its class

If it has both, pick whichever you want

BTW: Your themes css file is located in the sites/all/themes/YOURTHEME/css folder.

Finally, after you make changes always remember to clear/empty your cache