What sort of CD are you trying to burn? If it's an ordinary audio CD then thenoob1 is right. For audio CDs I don't even really think of file size at all; I just figure out if the total time of the files is under 80 minutes or not. Original file size matters not, be it mp3, ape or otherwise.
Now if you're burning an mp3/wma data disc to be played on a player capable of playing those sorts of files the answer is different. If you open up the disc burning dialogue and select "Data CD or DVD" you'll see a drop down menu open giving you options to convert to mp3, wma etc. Below that there's a link giving you conversion options which will allow you to set bit rates, etc. You can, for example, have MC down convert mp3 files with 320k bitrates to 192k. This will allow you fit more music on a disc.
But there really is no way to force MC to figure out an mp3 bitrate that will allow all the files you choose to fit on a single disc. MC will roughly figure out how many of the files you choose will fit on the disc, given the bit rate you specified under "Convert Options". It will allow you to burn your selected files across multiple CDs if necessary. But be forewarned; I do see posts here saying that MC has some issues figuring out how many files will fit a disc.
