Is there any limitation on the size of the requests / responses that can be successfully sent?

Yes. But I have no hard figure to give; it most likely will depend on the version of PHP in usage and its configuration.

Keep in mind that this library is not optimized for speed nor for memory usage. Better alternatives exist when there are strict requirements on throughput or resource usage, such as the php native xmlrpc extension (see the PHP manual for more information).

Keep in mind also that HTTP is probably not the best choice in such a situation, and XML is a deadly enemy. CSV formatted data over socket would be much more efficient.

If you really need to move a massive amount of data around, and you are crazy enough to do it using phpxmlrpc, your best bet is to bypass usage of the xmlrpcval objects, at least in the decoding phase, and have the server (or client) object return to the calling function directly php values (see xmlrpc_client::return_type and xmlrpc_server::functions_parameters_type for more details).