A full screen option has now been enabled in the new Gmail compose experience. To try it out:
1. Click Compose.
2. Click the double-arrow icon at the top right of the compose window.
3. Enjoy a larger composition mode that displays all of the formatting options by default.
If you want to make this option your default view, click into the options menu at the bottom right corner of compose and choose “Default to full-screen.” The next time you click Compose, you’ll be sent to this mode. Both types of compose can be minimized by clicking the black bar at the top.
For more information:
Google Apps for Business, Education, and Government