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