$SpamAssassin_SpamcMaxSize
to a size larger than 256k.
However, it isn't sending the value to spamc so spamc is still throwing an error on the emails that are larger than 256k.
According to http://spamassassin.apache.org/full/3.1.x/doc/spamc.html
The following flag needs to be sent when spamc is invoked.
-s max_size
Set the maximum message size which will be sent to spamd -- any bigger than this threshold and the message will be returned unprocessed (default: 250 KB). If spamc gets handed a message bigger than this, it won't be passed to spamd. <br/>
The size is specified in bytes, as a positive integer greater than 0. For example, -s 250000.<br/>
I rank this as medium, as it is not working and throwing a lot of errors on mail messages that are too big.
If more information is required please let me know.