The 66-character line is a starting point, not a law
Everyone quotes the same number: sixty-six characters per line, the comfortable measure for reading. Fewer people mention where it comes from — mid-century studies of typewritten text — or that the studies themselves say the comfortable range is wide, roughly 45 to 90 characters depending on the typeface and the reader.
What the number is really guarding against is simpler: lines long enough that the eye loses the return path, and lines short enough that the rhythm becomes a twitch. Both failures are visible from across the room. Squint at your page; if your eye jumps a line on the way back, the measure is too generous.
A comfortable page is one where the reader forgets the page exists. Every typographic rule is a hypothesis about how to get there.
Practically: pick your measure first, then your type size — never the other way round. The size that fills a 66-character line at your chosen leading is the size your page wanted all along.