I have been able to log into my .com wordpress site and did an export of only posts. File size was 5424KB. The error message I received was:
Sorry, there has been an error.
The uploaded file exceeds the upload_max_filesize directive in php.ini.
Yeah that's five megs. Most hosts default to about 7megs, but I can tell you from experience that an XML file that large is not going to import cleanly on most small, shared servers. I work for a webhost. 5megs is about the time I start telling people "You're gonna break PHP." I bet you're running out of PHP memory, but your host should be able to let you see the PHP error logs to see exactly what the error is
FWIW, it will cost you $129 to have WordPress.com do it for you :)
