MP3 should be small enough, however:
Settings tab>Apple>Formats>Audio>AAC128Kbps-
Then play around with parameters in inspector window (i.e. bit rate, sample rate) to decide how small you want it (To check out how your changes affect the size of your file always click on the first button/icon in the inspector window, which is SUMMARY. Please note that the estimated size is given in MB/Hour...You do the math with regard to the actual length of your track).
Cheers,
G.