He said it was the size of his pinkie nail. Not sure how big that is, but the half penny is 25.5 mm, while the 1/3 and half farthings from the 19th century were 15-16 mm. I have a 1923 whole farthing that measures 20mm.
The 1837 half farthing looks closest to me but actual size will probably determine it. If that's the size of your pinkie nail, you must have some large hands!