Do the -pageheight and -pagewidth options of the postscript command can help you?
From the canvas page of the Tk manual:
-pageheight size
Specifies that the Postscript should be scaled in both x and y so that the printed area is size high on the Postscript page. Size consists of a floating-point number followed by c for centimeters, i for inches, m for millimeters, or p or nothing for printer's points (1/72 inch). Defaults to the height of the printed area on the screen. If both -pageheight and -pagewidth are specified then the scale factor from -pagewidth is used (non-uniform scaling is not implemented).
-pagewidth size
Specifies that the Postscript should be scaled in both x and y so that the printed area is size wide on the Postscript page. Size has the same form as for -pageheight. Defaults to the width of the printed area on the screen. If both -pageheight and -pagewidth are specified then the scale factor from -pagewidth is used (non-uniform scaling is not implemented).
(I never used this)
HTH
ulis