How long can a single note be, and what do the size warnings mean?

Type as much as you like - for normal notes, size never comes up. As a single note grows very large, a small hint appears beneath it and escalates in three steps: around 50,000 words it notes the note is getting long and may lag on slower devices; around 75,000 words it turns amber, meaning editing may start to stutter; and around 100,000 words it suggests splitting the note because you are nearing the sync limit. These are guides, not hard stops, and nothing prevents you from continuing.

That sync limit is the only real ceiling. A single note can hold up to about 1 MB of text once encrypted - very roughly 120,000 words of typical English, and fewer with a non-Latin script or heavy formatting. A note past that keeps working and stays safe on the device you wrote it on, but that one note will not sync to your other devices (you will see a "failed to sync" notice). The rest of your notes are unaffected: one oversized note never blocks anything else.

The fix is easy: split a very long note into a few smaller ones. The content is identical, it syncs without trouble, and the editor stays fast. A live word count under each note lets you watch the size as you go. And if you were about to stress-test where the wall is, now you do not have to.