I tried to back up my home folder and kept getting:
"could not determine size of resulting image file".
I finally found the folder that was blocking the process (which contained many files).
Is there a system to identify the rogue file and its location?
thanks
ps
I asked on the ubuntu forum but did not get a clear explanation.

Unfortunately I do not know if there is. You could try enabling "kdebugdialog" and see if K3b outputs any debug output which hints at the file(s) causing it problems.