Source: http://www.engadget.com/2013/07/19/google-gmail-full-screen-compose/
It looks like those not satisfied with the recent Gmail redesign will soon have a new option to make things operate in a slightly more familiar manner. Google announced today that it's begun rolling out a new full-screen compose window that gives you more room to work with than the current option that's pinned to the lower right corner. What's more, you'll also find that the formatting toolbar is now on by default instead of requiring an extra click as it does now, and you can choose to make the full-screen view the default if you prefer it. If you don't have the new option already, Google says you should within the next couple of days.
Source: Gmail (Google+)