I'm current developing a WebService in a Bea Websphere 6.1 environment with Axis 2 implementation, I'm trying to respect the current standards for Web Services (JAXB, JAXWS, StAX...).
How long can be an XML message on this platform?
I've searched around but I haven't found anything related to this topic.
Don't know much about the limitations on you're platform, but in general, there are no hard limitations. Using Axis with the standard SOAP stack will take the SOAP envelope and create an in memory representation of it (dom or dom like), making you're heap size a limitation. In case of using very large XMLs, a better way would be to use STaX which is an XML streaming parser library. This would remove all limits from the size of the XML you may consume or create.
Yes, due to way Jax-ws & SOAP work, the whole payload gets bound: either to a Tree (higher memory usage) or to POJOs/beans (bit lower memory usage). Either way, memory usage will be proportional to message size.
Although theoretically it might be possible to create streaming/chunked interface for Soap processing, it is non-trivial to implement, and no stack currently does this.