I believe Google switched everyone over to the new compose a few days ago. If you click the little triangle in the bottom left of the compose window, to the rig

I believe Google switched everyone over to the new compose a few days ago. If you click the little triangle in the bottom left of the compose window, to the right of the garbage can, you'll see an option to default to full screen. I think that's as close as you'll get to the old compose experience.