Was the CD formatted for RW or as a CD R.
If it is formatted as RW then you have lost about 200 megs by the format procedure. If this is the case, your distro of 5.1 is about 696 MB's.
If the CD RW is 700 MB's, after formatting, it will be 500 MB's.
Not sure if you would receive an error in your burning program though. Most would see that the data would not fit and not allow you to continue burning to disc.

Your CD as first boot device, should be configurable under boot options or boot devices, some such. There are many bioses with different menu configurations. You can access everything in your boot menus, if you have made changes and are unsure, or just exploring, choose exit without saving. Wahlah, no harm done.