Another somewhat trite answer I can give is small enough and no smaller. When speaking at conferences, I nearly always ask the question who has a system that is too big and that you’d like to break down? Nearly everyone raises their hands. We seem to have a very good sense of what is too big, and so it could be argued that once a piece of code no longer feels too big, it’s probably small enough.
This is unique to each team and in the classic way, 'it depends'. Depends on the language, framework choice and the boundaries of responsability.
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