WordPress visual editor provides you a button called Distraction Free Writing Mode (Alt + Shift + W). It allows you to switch to full screen post editing page where it hides all unnecessary dashboard menus, widgets etc and shows you only content editor with toolbar but then also it has a fixed width and there is no option to get a wider area even if you are having a big screen device.
This article in on a simple WordPress plugin that enables full screen width editing direct from visual editor toolbar.
Install and activate FullscreenWidth plugin. After activation visit Add New Post page and switch to full screen mode by pressing its button or use key board shortcut (Alt + Shift + W). Now you will be seeing a new text box added to your toolbar, you just have to enter the width you want in your edit content text area and it changes instantly through AJAX.