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.
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...