I am not sure what you mean, The posts content takes the whole space of the site Content area.
The site width is set at its default 940px, so if your browser is larger it will have empty space on each sides.
You can change the Site width in Main options > Wrappers areas > Wrapper Area > Theme Width box.
I suggest you explore each on the option pages and check what these do so you can become familiar will all the formatting options.
You can put some temporary colors in the various area BG boxes to make sure you understand which is which.
You can also visit the demo site here: http://demo.weavertheme.com/ Each page explains something about what the theme can do
For your posts, go to Main options > Post Specifics and check what is there. For example, right now your Featured Image is set to be above the title and centered, but you have a lot of different choices in the Featured image section of that page.