With Mail full screen, do any of the following:
Make one side bigger in Split View: Move the pointer over the separator bar located in the middle, then drag it left or right. To return to the original sizes, double-click the separator bar.
Open a message: Double-click a message in the message list. Each message you double-click opens in a new tab.
Start a new message: Click the New Message button in the message toolbar (or the Close button Manage windows on Mac