This question already has an answer here:
Is there anyway besides using a 3rd party email application to revert GMail to the old-style compose and reply email boxes? The style that allows you to use your full screen?
It drives me absolutely insane when a company makes an 'enhancement' and reduces usable window space. If I am writing a long email I want to be able to use the maximum amount of window space and not be crammed into something that is 25% of my monitor size.