The editors all use 100% width so the width is determined by the container element width. So you can find the div containing the editor and adjust the width by CSS.
In some cases if the container width is too narrow the editor toolbar will be wider than the container and cause it to overflow.
One can customize the editor toolbars by configuration.
Often people find the contact form is too wide for the side column, one could customize the toolbar to make it smaller, but more easily, one can make the side column a little wider on the page where it is needed.
Hope that helps,