If you use Win 7 or Win Vista and Istaria is installed into the standard windows folder, for example C:\Program Files (x86) you need administrative rights to edit the file there. Easiest way is copy the file to desktop, edit the file you have on your desktop and move this file into resources override folder.

Here you will find a slightly enhanced version of setscale with instructions, so you can type as shortcut /ssc

Istaria Wiki Setscale