When the CD is full iTunes

When the CD is full iTunes automatically ejects the disk and tells you that either burning is complete (it has burnt the entire playlist to that disk) or it will tell you to insert a new disk because it can't fit all the playlist onto one disk. However often it will show you at the bottom (for eg) that you are only burning 400mb of mp3 to the disk, so you think "Oh it will all fit onto my 700mb disk".

But this is the cheeky part of iTunes. The default burn is 'Burn Audio Disk' which uses up an awful lot more space than mp3 so actually when your seeing a small file size your seeing it as mp3 not audio.

If you want to change the burn default from 'Audio Disk' to 'mp3 disk' go to Edit>Preferences>Advanced>Burning and click on mp3 cd. This should give you at least twice the amount of songs on the disk than Audio CD mode. However, bare in mind that not all CD players can play mp3 cds so check that whatever you plan on playing it on can play mp3 cd (most newish players can play mp3).

I hope this is of help to you. Feel free to ask more questions.