There are a few issues here.

First, you can control the size of everything on the site by tweaking your browser's font size setting.  If you have a mouse with a wheel, Internet Explorer and Firefox (and possibly others) let you do this by holding down CTRL and rolling the wheel up or down.  You can also get to it in their menus: In IE and Firefox: View > Text Size.

Second - we may not scale the width much larger.  This is a tough one.  The wider a page gets, the harder it is to read text.  From a usability point of view, very long lines of text are terrible -- most people can only read text about 80 or 100 letters wide before it starts to get hard to jump from one line to the next.  On a large screen, if we let the site scale to fill the full width of the screen, you'd have lines of text so long that you couldn't read them. 

Also, people with large screens might be putting gigantic photos into their posts, not realizing that to the 50% (or so) of visitors with 1024 x 768 or 800 x 600 screens, those big photos make the page so large that the text bleeds off the right edge of the screen even with the browser at full-screen size.

So, this is a tough one.  We're trying to make it easier for people to read posts, and trying to make it harder to accidentally create broken pages (with large images, etc.).  For these reasons, we might not change the default behavior.

However, I was thinking of making this an option in the site theme, so if you want your site to scale to fill the screen of every visitor, you can check a box to make it happen.

-Mike