If it's coming from woodstox like that, then you aren't sending it as an MTOM attachment. My first suggestion would be to flip it to MTOM so it can be handled outside the XML parsing. Much more efficient as we can keep it as an inputstream or similar and not have it in memory.
If you want to keep it in the XML, you can set the property: "org.apache.cxf.stax.maxTextLength" to some larger value. Keep in mind, stuff coming in from the stax parser like this are held in memory as either a String or byte[] and will thus consume memory.