Have you noticed the little triangular grouping of dots down at the lower right corner of the message text box? That allows you to greatly expand the size of your message area by dragging that down. It’s a little clunky, but you can expand the message text area to nearly the entire screen, if you so choose. My only gripe about doing so, is that the attach/save/send/delete icons can scroll off the top of the screen, if you enlarge it too much.
This is a huge improvement from the previous web mail, and any other web mail I’ve used (which admittedly, isn’t that many).