Largest Coin In The Us?

Besides the Yapese rai stones, I believe the largest circulating "coins" were the Swedish plate money: huge ingots of copper struck in Sweden in the mid-1600's. They were originally intended to be the backing for Europe's first attempt at introducing paper money, but when the banknotes were generally rejected, the plate money itself circulated. The briefly-issued 10 daler piece (KM# PM5) was a rectangular slab 64cm long, 33cm wide and weighing 9 kilograms.

I believe these plate money are the largest objects you can find in the Krause catalogues.

Back on subject... I am fairly certain that the largest U.S. Coin made was the 1877 Half Union $50. It was originally made of gold (2.5 ounces?), but other trial pieces were made as well, here is a copper example;

HBCC-6137 1877 $50.00 or Half Union. Liberty Head or Coronet. Copper. Judd-1547, Pollock-1720. Rarity-7.
Obverse Design: Head of Liberty to left, coronet in hair reads LIBERTY, 13 stars around, date below, small B, for William Barber, in field above date.
Reverse Design: Very similar to the standard reverse of the regular-issue double eagles of the era, central eagle with outspread wings, rays and halo of 13 stars above eagle, IN GOD WE TRUST within halo of stars, E PLURIBUS UNUM on ribbon that encircles eagle, UNITED STATES OF AMERICA FIFTY DOLLARS around.
Physical Description: Incredible splashes of rich violet, rose, and blue fairly leap from the surfaces of this beautiful pattern.
Provenance: From Stack's sale of the DiBello Collection, May 1970, Lot 510.
Narrative: The Half Union patterns of 1877, the most impressively sized of all United States patterns, were struck in gold and copper, and in two slightly different varieties in each metal. Evidently they were struck by late August 1877, as impressions from the dies were sent to H.R. Linderman on August 30 of that year. The denomination was only considered briefly before being shelved permanently.
Specifications: Weight: 757.0 grains. • Diameter: 51.0 millimeters. • Edge: Reeded.
Harry W. Bass, Jr. Commentary: Inventory number: 14224.

http://www.harrybassfoundation.org/....asp?id=6137