On the monitored instance, there is a small schema that is created, but only if you install the repository. The schema is used for staging advisories and change tracking before it is transferred to the middleware and repository.
The size of the schema is 50MB for data and 50MB for index, so 100MB total.
NOTE: This assumes you have a dedicated repository. If the repository is installed on the monitored instance (not recommended) much more space is required.