This presumes long narrow board, as it probably differs for wider boards. I spent more time on the pretty colours than the actual ratios, so would happily refine those numbers if anyone has any data or feedback?
What size did you find diminishing returns on the large side, and too small on the small side?
Spreadsheet here: docs.google.com/spreadsheets/d/15rVPnnlLZkhOkmEmCUAB3mst5VYilamjlVagCQrZhl0/edit#gid=343243571