That's just because the CDVD plugin doesn't has any funcion to get the disk size. And I don't know if it can be read from the disk data, so i had to use a windows function to get it (to get the total number of bytes and divide by 2048).
to parotaku: the path must exist. I forgot to put a check on that. I will make it ask if you want to create the directories if needed. I will also put an option to change the prefix of the videos (the program names them "stream_sector_offset.mpg" now).