The new Blogger Template Designer has made it very easy to change overall blog width as well as the width of the main column and the sidebars. All you need to do is to sign into Dashboard, click DESIGN > TEMPLATE DESIGNER > ADJUST WIDTH and you will be provided with sliders to drag and drop to whatever values your heart desires, then click APPLY TO BLOG to save the new settings (see screenshot below):
However with the Simple (and I believe the others too) template, the overall width is limited to maximum 1000px. So what do you do when you want your overall blog width to be greater than 1000px.?
Well, while in the DESIGN tab/mode, click EDIT HTML to open the template editor and search for these codes (use ctrl+ F and search for, say, <b:template-skin> or any portion of the codes below:
<b:template-skin>
<b:variable default='930px' name='content.width' type='length' value='1000px'/>
<b:variable default='0' name='main.column.left.width' type='length' value='0px'/>
then change that 1000px to whatever values you fancy and you would have changed the overall blog width.
BTW the default for the overall blog width is 930px which is roughly the value you would need to set in order to optimize your blog for 1024x768 screen resolution which is what many web designers are doing because the previous lowest denominator 800x600 screen is now used by very few people.