I bet you could add all the sizes you want before I could program it to put them in.  

  You can have up to 100 sizes in the Studio Edition.  The reason I don't include sizes like 16x20 and 20x24 is that those sizes won't fit on most printers unless you are running wide format (wider than 13 inch).  The result is that I give everyone sizes that 90% of the people will click on and get a poster, potentially getting people in trouble.  Instead, why don't you just create the sizes you want.  It takes maybe 15 seconds to add a size.  Just click on the tool button to the right of the size drop down and select "Add a size".  Do that for however many you need.
Mike