Open up ckeditor/config.js and put the following code in there.

CKEDITOR.editorConfig = function( config )
{
	config.resize_enabled = false;
};

Ultimately, I was aiming to disable only horizontal resizing but that feature’s not available yet (can’t seem to find the forum post where a dev suggested they might implement it). The whole reason I wanted to disable resizing in the first place is because as soon as you would start to drag the resize handle, the editor width would shoot out and you couldn’t make it any smaller. Turns out it’s another somewhat hidden config setting forcing the resize width to a minimum amount.

If you want to keep resizing enabled but adjust the minimum width, use the following code instead.

CKEDITOR.editorConfig = function( config )
{
	// Sizes are in pixels
	config.resize_minWidth = 550;
	// config.resize_maxWidth = 700;
	// config.resize_minHeight = 200;
	// config.resize_maxHeight = 500;
};

As you’ll notice, there is also a setting for resize_maxWidth, along with similar ones for controlling the height. Any config settings you put in this file will be the new defaults for all editor instances, and once you know some of the things you can change here, you can really streamline the setup of new editor instances throughout your application.