MHEG-5 is just another name for the ISO/IEC 13522 specification. This specification only defines a language, some classes and the encoding rules of MHEG-5 objects. It does not define details like the "size of the screen" or how the content is delivered to the viewer. These things are defined in documents called Profiles. Almost all MHEG-5 profiles for digital television environments are based on the profile defined in the UK D-Book.
Countries or Operators usually define their own profile to ensure that devices deployed into their market support the features used by their MHEG-5 applications.
Countries and Operators using MHEG-5