I know that this thread is a bit old, however for the sake of comunity knowledge - I would like to add, that the actual property which helps TN identify of a doc to be large is below - which you need to configure in TN through navigation - Administration > Integration > B2B Settings > Configure Properties
tn.BigDocThreshold
tn.xml.xqlThreshold
The property, you requested for - will provide location for where to place the inbound document - after it is recognized big by TN, when processing the document.
#webMethods#Integration-Server-and-ESB#B2B-Integration