The gtk_widget_set_size_request() function is used to set the size of a widget to a specific size. The function gtk_window_set_resizable() function sets whether the user can resize a window, which they can by default. The definition of these functions are:
void gtk_widget_set_size_request (GtkWidget *widget, gint width, gint height); void gtk_window_set_resizable (GtkWindow *window, gboolean resizable);