https://en.numista.com/catalogue/pieces7394.html
I changed the page now - I looked somewhere and saw it was 38.9mm
Other places said 36.5 / 37 / 38 / 38.1 / 38.5 / 39 / 40mm but 38.9mm seems about right - unless anyone else knows different. With it being issued for many years (1824-1897) and so many mint marks etc I guess a few size variations are expected. Incidentally, I counted the huge number of lines on that page, and think there are 886 on there - must be a Numista record.