#include <gtk/gtk.h>

int
main(gint argc, gchar **argv)
{
    GtkWidget *window;
    GtkPaned  *paned;

    gtk_init(&argc, &argv);

    gtk_rc_parse_string("style 'my_style' {\n"
                        "    GtkPaned::handle-size = 200\n"
                        " }\n"
                        "widget '*' style 'my_style'");

    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

    paned = (GtkPaned *) gtk_hpaned_new();
    gtk_paned_add1(paned, gtk_label_new("left"));
    gtk_paned_add2(paned, gtk_label_new("right"));

    gtk_container_add(GTK_CONTAINER(window), GTK_WIDGET(paned));

    gtk_widget_show_all(window);
    gtk_main();

    return 0;
}

change gtk.Paned handle size