Problem:
Since version 9.0.0 the max letter size to filter is hardcoded in the spam handler and cannot be changed easily [1]
Solution:
Write a wrapper script:
Rename /usr/bin/spamc to e.g. /usr/bin/spamc.org
mv /usr/bin/spamc /usr/bin/spamc.org
Write a new /usr/bin/spamc shell script:
#!/bin/bash
/usr/bin/spamc.org $* -s 512000