What counts against my storage, and how do I free space?

Everything you sync, at its encrypted size: note text, images, audio recordings, and file attachments. The bar under Settings > Storage shows the total; each note's own footprint is listed in its note options as "Storage used". Trashed notes still count until the trash is emptied.

To free space fast, open Files, sort by Size, and delete the biggest items - attachments dwarf text in almost every account. Then empty the trash: the Trash view shows how much space "Empty" will free. Text alone almost never fills a quota; even the free 50 MB holds tens of thousands of plain notes.