I would put the code in the applicationDidFinishLaunching handler.
Perhaps the following, to set the window to the current screensize.
set screenFrame to current application'sNSScreen'smainScreen()'s frame
set win to current application'sNSWindow'salloc()'s initWithContentRect:screenFramestyleMask:3 backing:(current application'sNSBackingStoreBuffered) defer:false
its setBackgroundColor:(current application'sNSColor'swhiteColor)
its setCollectionBehavior:(current application'sNSWindowCollectionBehaviorFullScreenPrimary)
its toggleFullScreen: (me)