gtk-window-set-default-geometry (window width height)

window -- a gtk-window widget
width -- an integer with the width in resize increments, or -1 to unset the default width
height -- an integer with the height in resize increments, or -1 to unset the default height

Like the gtk-window-default-size function, but the width and height arguments are interpreted in terms of the base size and increment set with the gtk-window-set-geometry-hints function. The gtk-window-set-default-geometry function has been deprecated since version 3.20 and should not be used in newly written code. This function does nothing. If you want to set a default size, use the gtk-window-default-size function instead.