The screen size I want to use in the Screens tool isn't there.

You can select more Screens templates and create your own from within xScope's preferences pane. Simply open the preferences by going to xScope -> Preferences. From there, select the Screens section. Here you can check on and off different screen configurations, and can also add your own custom configurations by clicking the + button at the bottom of the screen and entering the desired dimensions in for the new configuration highlighted in the list.