Is there a max size for POST parameter content?

As per this the default is 2 MB for your <Connector>.

maxPostSize = The maximum size in bytes of the POST which will be handled by the container FORM URL parameter parsing. The limit can be disabled by setting this attribute to a value less than or equal to 0. If not specified, this attribute is set to 2097152 (2 megabytes).

Edit Tomcat's server.xml. In the <Connector> element, add an attribute maxPostSize and set a larger value (in bytes) to increase the limit.

Having said that, if this is the issue, you should have got an exception on the lines of Post data too big in tomcat

For Further Info

There may be a limit depending on server and/or application configuration. For Example, check

There is no defined maximum size for HTTP POST requests. If you notice such a limit then it's an arbitrary limitation of your HTTP Server/Client.

You might get a better answer if you tell how big the XML is.