In reply to James Walker.

Hi James, for answer to your question please see my blog post How to split a raw file or a data set into many external raw files. If you know the size of your whole output, you can easily decide how many splits you need to keep each file under 20MB. You may also decide to write your output files into a zip file as you create it (examples are at the end of that post) - in this case you may not need splitting at all.