this link may be useful;
www.oreilly.com/.../ch04_08.html

it discusses the samba logging
configuration. The setting worth a look is
called "max log size"
"The max log size option sets the maximum size, in kilobytes, of
the debugging log file that Samba keeps. When the log file
exceeds this size, the current log file is renamed to add an
.old extension (erasing any previous file with that name) and a
new debugging log file is started with the original name"