set_handle_size() can currently be used to set the width or height of the handle in pixels. The default setting is 5 pixels in the win32 binary, or 10 pixels in the 1.2.* series of GTK+. In GTK2 the size of the handle will be set at 5 pixels, across both platforms.
Please note that both this function and the handle_size property are deprecated in GTK2, and so should not be used.