The first question we need to address in seeking a theory of topic-based information design is the perennial “how big is a topic”. Whether we are talking about the reusable blocks that DITA calls topics, or about Every Page is Page One topics that are sized for a reader, the question of size is always the first one that people ask.
In the discussion of Keith Schengili-Roberts’ blog post, What Size Should a DITA Topic Be? (the discussion is on a closed LinkedIn community, DITA Metrics, not the blog itself), Katriel Reichman suggested “one idea, one topic”. Another approach that I have used in the past is to say that a topic supports one task.