Unfiled is created by Spark when he doesn't know in what group to put contacts or group is not set while adding the contact. Sometimes it will show unfiled when you delete groups on the server and there are still online users, etc. Spark can't show users just in the roster without any group, so it creates Unfiled when he can't find a group.
I'm not sure about file size, but in default.properties file there is a value 10048 for maximum file size, so maybe it's 10 MB http://community.igniterealtime.org/docs/DOC-2163