turtle.screensize(canvwidth=None, canvheight=None, bg=None) 
Parameters:
canvwidth – positive integer, new width of canvas in pixels 
canvheight – positive integer, new height of canvas in pixels 
bg – colorstring or color-tuple, new background color  
If no arguments are given, return current (canvaswidth, canvasheight). Else resize the canvas the turtles are drawing on. Do not alter the drawing window. To observe hidden parts of the canvas, use the scrollbars. With this method, one can m