I would create a maximize variable then do something like this
mob/verb/Fullscreen
if(Maximized)
winset(src,"TheControl","is-maximized=false")
winset(src,"TheControl","can-resize=true")
Maximized = FALSE
else
winset(src,"TheControl","is-maximized=true")
winset(src,"TheControl","can-resize=false")
Maximized = TRUE