You can change 3 elements of the theme’s width right from the Theme Options page, these are: “Content Width” (the width of the main article, not including the sidebar) and “Sidebar Width” (the width of the left or right floating widget area), and “Padding” (the distance between these elements). All measurements are in pixels, so you only need to type the desired numerical width in the text boxes.
The entire page width is equal to the combined widths of the Content + the Sidebar + the padding on each side of each element.
To set these widths to your own custom values, first open the Theme Options page. This item is under the Admin Bar when on the public site (if enabled) and is always available on the dashboard under the Appearance tab, or at. It should look like the screenshot below:
Then enter the number of pixels you would like your new theme widths and paddings to be and click the “Save All Changes” button at the bottom of the page. Take a look at your pages and determine if the widths need further adjusting!