If you are using Page View, where when you scroll down you see page breaks, then you can adjust the size with the standard print settings for the project, with File ▸ Page Setup....
Otherwise, for the default view, regular continuous scrolling, have a look at this thread. In this case you probably want to leave fixed width enabled, but adjust the width setting. An easy way to do that is to make the editor area as wide as you want the text to be, and then click the Use Current button. It will fill the editor at that point, but go no wider if you make the window larger again.