I can't resize the text INPUT area on Messenger. Used to be able to. Is there a registry setting to change?

This is intensely irritating. I need a larger input area, certainly more than just two lines, but cannot find a way to resize this portion of the chat window. If anyone who can advise on what I must to to re-enable this functionality which used to be present in version 7 and, for a short while at least, in version 8, I would be grateful.