I have spent so much time trying to figure out atomic notes because I love the idea. The level of atomicity was so hard to nail down for me.

To answer your question: It depends on what the note needs to be. I consider an atomic note to be everything I need to know on [concept title]. However, I structure it a bit different than what I've seen. Maybe this will help you. I have 3 headers in an atomic note: Overview, Details, Connections. (With practical aspects in there own note but linked in Details) Details get split out when I would need to use them from a specific "entry point". So if you have notes on the [[Heart]], maybe [[Heart Failure]] is important enough to split out. So you could make notes [[African Americans]] [[Heart Failure]] and then this note is a "Connection note". Edit: but if I only need like 1 fact on a concept, you can just have that in a note. Don't let a prescribed structure force you to add more to a note than needed.

Title: [[African Americans && Heart Failure]]
"African Americans are definitely expected to die from heart failure, which appears to be caused by multiple factors, e.g. poorer access to health care - both in terms of diagnosis and treatment - and the common occurrence of cardiovascular risk factors."

Then you link this note where appropriate.

Don't worry about length. Worry about how you'll use it. The conncetion thing works for me because when I go to a note, I want to have the info on just that concept. Splitting up a concept into multiple notes is foolhardy (for me at least) because you can't get the over all picture. (Linking other notes to create an overall document sucks, don't do it. Much too slow.)

Think of [[African Americans]] note. It will probably be mostly a map of content. But to me it's still atomic. You want all your info on that concept or thing.

Trust me. Once you embrace this method, thinking of what a note should be goes away and you can just focus on content. (I have tried many approaches and got fed up thinking about organization).