F-SECURE ANTI-VIRUS FOR MICROSOFT EXCHANGE 7.10 Administrator's Manual page 262

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Max Message Size Specify the maximum size (in kilobytes) of
messages to be scanned for spam. If the size of
the message exceeds the maximum size, the
message is not filtered for spam.
The default value is 200.
Since all spam messages are relatively small in
size, it is recommended to use the default value.