I have a MBA (m2011 - Yosemite) with an external monitor setup (Dell 2007FP). I would like to use the Dell monitor in vertical mode by trying to rotate 90 degree in Display setup but the screen stay in horizonal resolution (black area above and below).
Depending on your version of Mac O/S there should be controls in "Displays" that allow you to rotate the display. On the Snow Leopard mac I have at work there is a menu to the right that says "Rotation:" and a menu with Standard, 90, 180 and 270 degree selections available in it.
I seem to recall it looks a little different in newer versions of Mac O/S