I don't even. CD-R's are designed for MP3 files, JPEG, and PNG files. You need a DVD-R, or DVD-RW in order to burn an ISO file.
Ubuntu used to be smaller then CD-Rs until recent versions, I believe 12.04, and people used to use them for Ubuntu.
Anyway, unless you want to try older versions, you need to get DVDs or a flash drive, I recommend flash drives since it's easier and you don't have to worry about messing up when you burn it, also you can use it more then once.