Still need scrollbars for bigger files here.
They're of a minimum manageable size on my 1680/1050 pixel desktop screen but did try tinkering by changing the value of the two separate lines :
GtkRange::slider-width = 26
GtkScale::slider_width = 26
from within file main.rc obtained by the command :
sudo gedit /usr/share/themes/Adwaita/gtk-2.0/main.rc
Made no difference though.
Had a quick look at some of the other Gnome gtk+ theme offerings such as Albatros, Bluebird, Clearlooks-phenix, EGTK, Greybird and HighContrast but only Greybird seemed noticeably wider.
Similar further tinkering with the corresponding Greybird file did not help.
Googling topic shows scrollbar width can be changed on other distros, such as Ubuntu's Ambiance, see this thread
, although the folder ~/.config/gtk-3.0 is empty here.
Suggest switching to another Desktop Environment such as KDE perhaps, Gnome has never offered much of a choice that i can recall.