A small copper coin, about a sixteenth of a day's wage — the price of two sparrows.
Wage
about 45 minutes' work (about 45 minutes' work).
Reckoned from sixteen to the denarius.
Diameter
1.6–2.2 cm (0.63–0.866 in).
Reckoned from surviving provincial bronzes, which varied a good deal from mint to mint.
An assarion is small change, roughly a sixteenth of a denarius. It is in the gospels to price the cheapest thing in the market: two sparrows for an assarion, and Luke has five for two, which is the discount you get for buying the fifth one nobody wants. The point being made is not about birds. Older English versions call this a farthing — and call the quadrans one too, which is why neither is filed under that name here.
Sizes are the range of surviving examples, and coins of one name varied from mint to mint. Bronze paid for small things and silver for wages and taxes: the widow gave two lepta (Mark 12:42), the temple tax was a didrachma (Matthew 17:24), and a stater paid it for two (17:27).