Since it looks like you're not using Opera, but instead something called Safari, aren't you able to drag the lower right corner to extend your textarea area?
(If you're using the super fantastic Opera, it's just the bottom bar of the textarea.
Though why you would want to type with just three visible lines showing, hey to each a peach.)
But then this would only solve your case and not across the board.
Haven't been of the use of looking at the preview text and thinking it was too much of a draw card. Used to be the preview had a dark grey background, but now it's got this nicely dotted border instead.
Is it really that much of a problem to be able to distinguish what the edit area is and where the preview of what you're typing up is?