It seems that composing an email in Gmail is not big enough, that little box that opens up in the bottom right hand corner just does not seem to do it for some people! Well, as usual Google have been listening to users feedback and as result will be rolling out a full screen option of the compose page! The user will need to enable this feature by clicking on the expand button.
According to a post on Gmail on Google Plus;
The new compose now has a full-screen option. After Gmail’s new compose rolled out to everyone in March, many of you sent us requests for features you’d like to see added. We listened carefully to your feedback and as a result, the new compose now offers a full-screen option.
Source: Google Plus