To display the percentage of the size of selected files relative to 4.7 GB (the size of an empty DVD), you would specify {cp+V=sb/4.7gb} .
Note that it will always only be an approximation. It is hard/impossible to predict exactly how much space files will take up when copied to another drive, due to unpredictable filesystem overheads. But unless you get very close to the limit or copy a huge number of tiny files, the overheads are usually negligible.