Is there a way to resize Safari windows while in full screen mode on Safari 6 or higher? When full screen mode was first introduced the window in Safari was resizable but it seems that it has been disabled or at least disabled by default. Is there a way to bring back this functionality?

http://osxdaily.com/2011/11/04/resize-safari-windows-while-in-full-screen-mode/
Safari Responsive Design Mode

This is more of a work around than a solution. Safari's Responsive Design Mode new to El Capitan and allows you to set the width of a website while in full screen mode. It is enabled through Safari's developer mode.