I have a couple ideas that might help with getting the main content area sized the way you want.
First, try using the menu button (top left) to show or hide or the left sidebar – the main content area should resize to take up the space, and you can experiment with making your browser window wider or narrower to get your preferred size.
Second, try making the window narrower, with or without the left sidebar open – when the window is narrow enough, all the content in the right sidebar will be send to the bottom of the page.
Third, and this suggestion is really more about finding the right number of words per line to make reading more comfortable, try different zoom levels. In most browsers, pressing control (or command on a Mac) and then + or - will zoom in/out. control and 0 (i.e. zero) will set the zoom back to 100%.