This command outputs the length of an iso9660 file system that is contained in given file. That file may be a normal file or a block device (e.g. /dev/hdd or /dev/sr0). In the absence of any switches (or errors) it will output the size of the iso9660 file system in bytes. This can now be a large number (>> 4 GB).
The size of the file (or block device) holding a iso9660 file system can be marginally larger than the actual size of the iso9660 file system. One reason for this is that cd writers are allowed to add "run out" sectors at the end of a iso9660 image.