The TinyMCE editor should behave responsively by default with the latest version of WordPress. (Try it out with e.g. the Twenty Fifteen or Twenty Sixteen theme and you should see the TinyMCE pane resizing and its toolbar wrapping to fit the available window.)
If it’s not resizing on your installation, then there’s probably some CSS in either your theme, or in a plugin that you’re using, that is overriding or interfering with the default TinyMCE responsive styling.
If you want to try tweaking your theme styling to fix the issue, this post might help:
How to make TinyMCE responsive
Please note also that enabling the extended editor (full set of buttons) can cause problems with some themes, as mentioned on the Forum Beginner Posts options page.