Currently the :REGISTER_VARIABLE function is limited to 1023 characters:
Important! Variables that are registered with :REGISTER_VARIABLE are limited to 1023 characters. Variables that are longer are truncated to 1023 characters without notice.
This means that if a variable is larger than 1023 character, then it will have to be split up, registered as multiple variables in Automic, and then pieced back together in Automic.
Please enhance this in future versions so that :REGISTER_VARIABLE does not have this limit.