amaguk,
This is like the knapsack problem, but not quite. The difference is that you don't need to hit an exact target, you just need to not waste any more CDs then an exact target.

For instance, you have 2GB worth of files. A perfect solution would have that fitting on 3 CDs with room to spare. As long as your solution doesn't require 4 CDs - you have sufficiently solved the problem. I got into a heated debate on this exact same problem in IRC some time ago and could have swore that I posted about it here - but can't find it. There is a recent similar thread (Burning ISOs to maximize DVD space), which mentions Algorithm::Bucketizer which I haven't tried myself. I would attempt the following:

• $buckets = ($total_size / 700) + 1
• Order files by size in descending order
• Round robin files (1 per bucket)
• When you encounter first file that will not fit, stay with that bucket but continue down the list until you find one that fits
• On the next bucket, start back at the top of the file list
• Wash, rinse, repeat

I am pretty sure the method will work. I was going to test it but the person complaining in IRC wouldn't provide a list of file sizes for me to try it out on and I wasn't motivated enough to make some up.

Update: As pointed out below, perfect solutions that exactly match (or even very nearly match) a whole number of CDs will wind up costing you 1 extra CD. That's why the +1 as the first bullet.