You need to first of all take a look at you blog using different browsers because when I see your blog with Google Chrome and IE8, I see a little different rendering of your text.
In any even, I'm not sure you have chosen a good template for what you want. In any event, you can edit the "#wrap" section of your template to control the width of the different elements. Right now you have a minimum width of 740 pixels and a sidebar that takes up 200 pixels. Ideally, a user who stretches his/her browser window would see the blog column stretch out to fill up the window while the sidebar stays the same. That is not the case with your blog. The sidebar stays the same and the text stretches to a point (the max width is set to 890 pixels) and when the user goes past that point, you just have a big blank with your backround image.
So, you need to experiment with those parameters -starting with the #wrap width of 740px. If you make that, say 700px, you will get a narrower column of text, but the problem with the empty space will be worse.
It sound more like you need to make your template a fixed width or find a new template. Fixing what you have would be easy if you want to learn about editing templates.