WordPress has a $content_width variable that allows you to set the maximum allowed width for any content in the theme. Although this is mainly used on the front-end, in the back-end TinyMCE editor, you can use the following css in your editor-style.css file so that you can set the editor width to match the front-end content width.

html .mceContentBody {
	max-width: 900px;
}

The editable area in the Gutenberg blocks are currently only 608px wide, which, when used on a large monitor, are pretty tiny. It would be good to see the editor make use of the $content_width variable so that we can accomplish the same as the current editor.