If a cd-r holds 700mb of information and you copy a file that is only 100mb of information can you continue to copy to a cd-r until all 700mb have been used?

It depends. If you do an "all at once" you will only be able to put the 100mb on. However, if you do a mulitsession burn, you will be able to keep filling up the disk until it is full. The downside to this is that not all computers can read these disks, so it is generally better to do an "all at once".