Full screen views of emails in Outlook

I keep Outlook in full screen view, but when I am composing an email, the message window is too small. There seem to be no option to make the message window full screen (the green button at the top left is greyed out), and hovering the mouse over the sides of the window doesn't give any ability to resize as with other windows on the Mac.

The only solution seems to be to exit full screen for the app as a whole, and then the message windows can be made full screen (the green dot reappears). Is this correct or am I missing something?

Thanks