you set it to burn in audio mode. this allows the cd to play in any cd-r or cd+r player or whatever cd you have. with audio cd's it is not the size that counts but it rather acts like an old record. it is the length of each song that yuo need to take into account. somewhere on the cd it should say the length of the cd. you could have 4 x 1mb songs each lasting 4 hours but you would not be able to burn a single one onto an audio disc. if you have an mp3 enabled cd player then burning in mp3 format would allow you to put on 700mb regardless of the length of each track.
Also MP3s are compressed, when they are converted to cd audio tracks, they are about 10 times bigger, hence why only seventeen will fit onto a cd.
Created a data cd and drag the MP3s onto it. But your CD player will have to be able to play cdrs and mp3s.