Go to Dashboard --> Template --> Customize --> Advanced --> Add CSS --> paste the following code --> Press enter after the last character of the last line } --> Apply to Blog.
Adjust the values accordingly. It is important for the two values to agree to each other (they must be the same). Keep in mind that if you set your sidebar's width to be too large, you'll end up with very little room for your content area. This is a nightmare for viewers with small-screen display as there wont be any ways to view your posts. Use it with caution.