I would like my text stimuli to be presented at the center of the screen, both vertically and horizontally. My script for this text is pasted below. Why isn't it being presented at the center of the screen? (It is being displayed at the top center--so it is centered horizontally but not vertically). 

What isn't centered vertically is the item text inside the bounding rectangle defined by the /size. Observe the difference when definingall of which you can find documented in the language reference for the <text> element.