Oh, you must be talking about backing up downloaded music, because if they were audio CD rips, you would probably have the origionals knocking around.
Just view the total size of the folders. Drag and select the relevant ones, right click, properties. Your 74 minuite CD can take 650 Mb, and an 80 min can take 700.
A complete MP3 album is usually 30mb or so, so your not going to have any space problems. Use whatever burning software you have, and create a data CD, rather than an audio CD, and just dump the files on like you would with any other data files.