Thanks for the response. It never managed to add the file via the webpage save, however when I set it to index the location instead it worked with no issues.
I don't think there is a specific maximum file size, beyond the underlying operating system / filesystem limits. You might find use in amrit's filestatus tool, which gives a realtime view into Splunk's tailing processor. See http://blogs.splunk.com/2011/01/02/did-i-miss-christmas-2/ for where to find it and how to use it.