<p>what configuration needs to be tweaked, and where does it live, in order to increase the maximum allowed post size?</p>

Apache Tomcat by default sets a limit on the maximum size of HTTP POST requests it accepts. In Tomcat 5, this limit is set to 2 MB. When you try to upload files larger than 2 MB, this error can occur.
The solution is to reconfigure Tomcat to accept larger POST requests, either by increasing the limit, or by disabling it. This can be done by editing [TOMCAT_DIR]/conf/server.xml. Set the Tomcat configuration parameter maxPostSize for the HTTPConnector to a larger value (in bytes) to increase the limit. Setting it to 0 in will disable the size check. See the
Tomcat Configuration Reference
for more information.