Thought I remembered a way you could change it, but not that I can find - that location is programmed in... and should be:
C:/Documents & Settings/Your Name/My Documents/Applications/DVDFab/Log
Didn't spot, on my quick peek, where you could limit the size of the log, either, but my glasses are dirty... just take a peek at common settings, each option one at a time - you may spot it.
P.S. How can we tell I don't go there very often?