Hi Robert,
the maximum file limit depends on the available RAM and hard disk space, and on the operation you attempt to do. If you limit yourself on editing XML in the text editor, Stylus has been tested on files greater than 1Gb; also previewing an XQuery using the DataDirect XQuery processor is scalable to gigabyte-size XML sources, provided that the query avoids constructs the prevents the streaming mode (e.g. the preceding:: axis).
Running an XPath query in the XML text editor, or switching to the tree view will require the building of a DOM representation, and this would surely not work.