Merge mode does not allow you to just make huge files. The Copy modes you are trying to use can produce only compliant type output, single or dual layer sizes for DVD or Blu-ray. Anything larger, it will compress to a compliant size within certain limits. No where nearly enough to put 8 films in a 'legal' size ISO, although you can use the Ripper module to make and then merge that many titles depending on the output size of each one. The Converter module does not support ISO sources, only video files.
