it all depends on the duration of the audio.
a rough estimate
1 min audio uncompressed (like wav or aif) = 10MB
1 min audio compressed (mp3, ogg) = 1MB
but with ogg you can bring the compression down with quality and make the file sizes smaller.
http://www.vorbis.com/faq/#quality
you have to play with this setting.. but you are probably not going get much smaller than a 1MB/per min and sound good. Depending on what it is you might be able to get down to 900/800kb per min...
Thank you for your help.