Greek στατήρ. Also called Tetradrachm, Piece of money.
Four drachmas in one coin — exactly the temple tax for two men, which is the point of the fish.
Wage
4 days' wages (4 days' wages).
Reckoned from four drachmas at a day's wage each.
Mass
13.5–17.2 g (0.476–0.607 oz).
Reckoned from surviving Tyrian and Antiochene tetradrachms.
Diameter
2.6–2.8 cm (1.02–1.1 in).
Reckoned from surviving shekels of Tyre.
A stater is a four-drachma piece. It matters because of the arithmetic: the temple tax was two drachmas a head, so one stater pays for exactly two people. When Peter is told to take the first fish he catches and open its mouth, the coin he finds is the one coin that settles the bill for himself and his teacher together, with nothing over.
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).