In the "Editing" section of Special:Preferences, under the "Editor" heading, there is a box labelled "Columns", by default set to 80. This setting determines the value of the "cols" attribute of the <textarea> element in the editing area. However, since textareas have CSS "display: block; width: 100%;", a change in the cols attribute has no impact.
I recommend removing the setting from the preferences screen.