One time, using the Linux tools and both Rockridge and Joliet extensions for the filenames (All horrible long filenames with spaces and crap...every file) I burned some 690 + Mb of files to a Verbatim 700 Mb DatLifePlus CDR disk.
bash-2.05$ du -s /mnt/cdrom 711431 /mnt/cdrom
Value is in kilobytes. I was kicking myself for not doing a test burn (-dummy mode, it's called in my software), thinking for sure I was going to make a coaster as it was displaying progress, but nope! There must have been extra space on that disk. I can't believe how much data I crammed on it.
I don't think I'll try my luck again like that, without testing first though.
Your CD burning software should let you know when you have filled up the disk, no free space is required on a CDR since it wouldn't act like a hard drive.
Use the test function before you burn, if there's a problem it should pop up then and give you a chance to make changes.