By default the editor’s width is set to 100% this calls the editor to atomically grow and fit different browsers sizes. As different configuration vary it is possible that you many want to define the width in pixels!

Visit the back end (administration section), go to ‘Extensions’ on the top menu bar and then clicks on ‘Plug-in Manager’. Scroll down and click on the ‘JoomlaCK' or 'JoomlaFCK ’(depending on the version), once opened you will be able define the width property in parameters on the left-hand side.

To apply the setting and close click on 'Save'.