To enlarge scrollbars and sliders for my fatty fingers, I added to ~/.config/gtk-3.0/gtk.css the following settings:

Code:
scrollbar.horizontal slider {
  min-height: 12px;
}
scrollbar.vertical slider {
  min-width: 12px;
}
If you want steppers, add the following to the same file:

Code:
scrollbar {
  -GtkScrollbar-has-forward-stepper: 1;
  -GtkScrollbar-has-backward-stepper: 1;
}
Only GTK+ 3 programs honor these settings.