When setting the pos and size parameters for a window, the window size can no longer exceed the bounds of the screen, and the window cannot be moved entirely outside of the screen. As a result of this, for clients running version 436 or higher you can use the following winset() to make a window full-screen:
if(usr.client && usr.client.byond_version >= 436)
winset(usr, "main", "pos=0,0;size=5000x5000")