2,957. How Big a Coin?

Hilchos Arachin Vacharamin 2:9

If someone commits to donate a silver coin, he may not give less than a silver dinar. If he commits a copper coin, he may not give less than a silver maah. [A maah is one-sixth of a dinar; see more here.] If he commits iron, he may not give less than one cubit by one cubit of iron suitable for the blade that kept ravens off the roof of the Temple, as we will discuss IY”H in Hilchos Beis Habechirah.

Hilchos Arachin Vacharamin 2:10

If someone commits to donate silver or gold without a coin, he must donate a hefty slab of silver or gold until he expresses that such was not his intention. Similarly, if he explicitly stated a specific weight of precious metal, but he forgot how much, then he must give until he says that such was not his intention.