You can configure a log size. This article shows you how:
How to debug the Symantec Endpoint Protection client
http://www.symantec.com/docs/TECH102412
NOTE: When using the default limit of 256 KB, the log file can rollover in a short period of time. Because of this, you may need to adjust the log size limit to a higher value (i.e., somewhere between 20,000 to 100,000 KB.) To modify the log file size limit, add the following "Log" key and "debug_log_filesize" value: [HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Symantec Endpoint Protection\SMC\Log]
debug_log_filesize=dword:00004e20
In the above example, the value of "debug_log_filesize" is the maximum amount of space (measured in bytes) that the "debug.log" file can consume. The number is written in hexadecimal (i.e., 00004e20 = 20,000 KB). The Symantec Endpoint Protection user interface allows an upper limit on the log size of 100,000 KB. If necessary, you can force the value higher by setting it here in the registry.