Resolution:
Yes, you can change either of these two items.  Do do so, you will need to manually edit the TIBCO Repository data file located at the root of the portal directory.  In the file &ltportal-dir>\TIBRepo_PB.dat, search for the string "portal.log"  (without the quotes).

It will probably be the 3rd occurence of the text- but it is the only entry in the TIBRepo_PB.dat file that has the full path to the file, such as "C:\tibco\portal/Logs/portal.log" (with the quotes).

Here is a sample extract from this file :
---------------------------------------------------------

&ltassoc name="fileSink">
                      &ltassoc name="role">
                        &ltstring name="name" value="Fatal"/>
                      </assoc>
                      &ltassoc name="role">
                        &ltstring name="name" value="Error"/>
                      </assoc>
                      &ltassoc name="role">
                        &ltstring name="name" value="Warning"/>
                      </assoc>
                      &ltassoc name="role">
                        &ltstring name="name" value="Info"/>
                      </assoc>
                      &ltassoc name="role">
                        &ltstring name="name" value="System"/>
                      </assoc>
                      &ltstring name="fileName" value="C:\tibco\portal/Logs/portal.log"/>
                      &ltstring name="fileCount" value="5"/>
                      &ltstring name="fileLimit" value="20000000"/>
                      &ltstring name="appendMode" value="true"/>
                      &ltstring name="name" value="FileSink"/>
                    </assoc>
---------------------------------------------------------

In the above, the setting with the name "fileCount" is set to 5.  This can be changed to a higher value if more backup files are needed.  These files will rotate, and will be named portal.log, portal.log.1, portal.log.2 ... portal.log.{fileCount-1}.

The "fileLimit" is the byte size of the the individual portal.log.n files.   The above value sets the file size to ~20 MB for each log file.

NOTE:  The log file settings in this file are read during the portal initialization process.  The portal webserver/servlet engine must be restarted for these changes to take effect.