On Friday Google decided it was time to give Gmail users their full-screen compose window back.
As the rollout of the "new" feature continues, you'll eventually see the icon to expand pop-up compose window. Clicking on it will reveal the full-screen compose window.
Now for the best part: you can set the full-screen window to appear by default going forward. To do just that, click on the More Options arrow in the lower-right corner of the compose window.
From here click on the "Default to full-screen" option, placing a check mark next to the listing.
You're still able to launch a, or window by holding Shift and clicking on Compose, or the expand icon.
Of course, if you decide the smaller compose window is better for your needs, you can simply replicate the steps above and remove the default setting.