We’re in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
How do I change the size of my Zotero pane?
You can toggle between a full screen and a smaller screen by clicking on the toggle icon on the right side of the Zotero pane. You can also click and drag your Zotero pane to resize it. You can place your Zotero pane either at the bottom of your page or at the top by clicking on the Zotero gear icon and selecting Preferences.