Sorry, folks - it is as Corinna mentioned above: Karissa ran a vBulletin patch recently, and the smaller Quick Reply box was a direct outcome - we don't know why. It functions (that's the main thing) but the dimensions were buggered. She will have to manually adjust it, if that's even possible - and it may be a while before it's resolved. In the meantime, you might adjust it as best you can with the methods described between Matt and LP.
All other functions appear to have been unaffected by the patch. Sorry for the inconvenience!