Hi. I'm new in lxde, I've just installed lubuntu 12.04, but I can do this in unity and I think this tweak is the same for both desktops: you have to edit your gtkrc theme file, in my case Lubuntu-default
sudo gedit /usr/share/themes/Lubuntu-default/gtk-2.0/gtkrc
and raise (or add) GtkScrollbar::slider-width = 25 and GtkScrollbar::min-slider-length = 80. It works for me, please confirm me if it works for you.
PD: I wrote a detailed howto tweaking for tablets (for ubuntu 11.10 with unity) in ubuntutablet.blogspot.com, I'll try to adapt all the tweaks to lubuntu.