Gmail new compose has accommodated a full-screen option.

In its efforts to make Gmail, a place where, everyone can easily organize and manage its contacts, Google has released a full-screen option for Gmail's new compose. We may recall that, this release
follows the Gmail's new compose announcement in March 2013.  
Henceforth when this option is enabled, the compose window is centered in your inbox and expands to fit on your screen.  

Connnectikpeople has also observed that, the formatting toolbar is on by default.  

Now, you can click on the expand button in the top right to switch to full-screen or set full-screen as the default by selecting Default to full-screen in the more options menu in the bottom right.