Wider content width in Simplr theme

Hi, total newbie here. I’m using Simplr theme (http://wordpress.org/extend/themes/simplr) and I can’t figure out how to widen the width of the main content area, so an image can appear in true full-size. The blog is here:
http://blog.andrew-fish.com

I tried inserting a line of code into the functions.php file:

$GLOBALS['content_width'] = 800;

I changed the width to 2000, but the page remained the same. I also had no idea where in the code to place it, so I put it under:

function simplr_globalnav()

I also tried a few other places. Does anyone have suggestions as to how to widen the width so I can show images full-size?

Also, I’m guessing that I’m using WordPress 2.6.2, because of the source code that comes up on my live page:

<meta name="generator" content="WordPress 2.6.2" />

Plus if you like Katey Sagal or Futurama / Married with children, you might like the article 🙂

Thanks for your help!