Whether you like it or not, the new compose window for Gmail is here to stay. Google announced in a Google+ post that the compose window is finally rolling out to everyone and the old full-screen compose window is out the door (or should it be out the window?).
But you can still get the full-screen experience when writing your emails. You just need to click on the expand button at the top right of the new compose window and set that as default by clicking “Default to full-screen” in the More options menu at the bottom right corner of the window.