Take a look at the LiveVideoStreaming example. You can comment out all the video lines and it becomes an audio streaming. You can then use “live-record” SteamType in Application.xml, or you can use LiveStreamRecord package:
https://www.wowza.com/docs/how-to-record-live-streams-httplivestreamrecord
There is a method for checking size of the file being recorded to here:
http://community.wowza.com/t/-/33816#4
Richard