The role of topics is discussed in the TIYP supporting material. See, e.g., the "Subject-based Filtering" section in https://reference.swim.aero/technical-infrastructure/guidance-for-pub-sub-push-implementation.html that discusses topic enumerations and topic trees. The getTopics operation allows a service consumer to request the list of topics that are available to use in the subscription. The topics themselves are URIs and should be based on the data that is exchanged, a common semantic reference such as the AIRM and the list of service categories.

How do I decide on the granularity of a service?

Granularity can mean two things: the size of the service and the size of the data it exposes.

The size of the service is based on the operational needs of the service consumer. The service should solve a whole problem for users and it is important to understand what that problem is e.g. is it to see all of the meteorological weather products or simply METARs. See the Service Orientation knowledge article - in particular the Identify and Design steps are useful in understanding users and their needs.

The size of the data exposed (e.g. get all messages, get an individual message, get a property of a message) is also driven by operational needs and the information exchange requirements.  The Interoperability Architecture, in particular, the organisational view is most relevant here.