Play with this CSS - strongly recommend doing this on a test server or local copy of the site:
margin: 0 5px 0 15px;
You can increase or decrease the width.
However, since you have thumbnail images associated with each post, you'll need to change those dimensions too. I can't tell you off the top of my head how to do that, it depends how your theme is set up. Is this a theme you downloaded from somewhere or did you create it from scratch? Did it come with any support or documentation? Often the theme maker will tell you how to change things like this more easily.