CKEditor window can be resized if the resize plugin is enabled. You can however define the resizing directions in order to have more control over the resulting editor appearance.
By default CKEditor resizing is allowed in both directions — vertical and horizontal. This is achieved thanks to setting the resize_dir configuration value to both (this is the default setting).
config.resize_dir = 'both';
If you want to allow vertical resizing only, you need to set the resize_dir configuration value to vertical.
config.resize_dir = 'vertical';
If you set the resize_dir configuration value to horizontal, CKEditor window will only be resizable in horizontal dimension.
config.resize_dir = 'horizontal';