The packetSize attribute of the AJP connector has been backported from Tomcat 6 into 5.5.21 but it is not documented on http://tomcat.apache.org/tomcat-5.5-doc/config/ajp.html. Neither is it documented for Tomcat 6 (see http://tomcat.apache.org/tomcat-6.0-doc/config/ajp.html).
The has been fixed in trunk and proposed for 5.5.x (and 6.0.x).
This has been fixed for 6.0.x and will be in the next release.
This has been committed to 5.5.x and will be included in 5.5.27 onwards.