Which box are you dragging? The one with the blue border, the outer one works on my machine.
Not too sure what you mean by Word not letting you make it bigger. If you type your message in a separate Word document page then it is A4 size and you can re-read, and check the whole thing before copying and pasting into the message box. You can then scroll up and down and read again what you have written, and amend if required.
Happy New Year