An idea should be a single point developed over a short format. Personally, I prefer 2-3 paragraphs. It states the claim, explains what it means and gives enough context that it makes sense on its own, without forcing the reader to jump elsewhere to understand the basics.
It is deliberately not a one-line fact. A note that small has nothing to argue with, and the reader has to assemble the meaning from fragments. The overhead associated with finding the idea is too high. On the other hand, a large note that deals with multiple points hides the connections that this format exists to show.
The test is whether the note could be asked a single question. If a reader would want to ask two unrelated things of it, split it. If a reader's question is only answered by combining it with another note's sentence, merge them.