Size and Value
Gold nuggets vary in size and are measured by weight. The most commonly found sizes weigh less than 1 ounce. As the weights go up, rarity increases, as well as value. A very large gold nugget can sell for thousands of dollars.
Identification
Gold is more malleable than pyrite, which is also known as fools' gold. Real gold nuggets are easily dented, and will bend, rather than break. Gold nuggets are the same color throughout: yellowish. Pyrite changes colors when you look at it from different angles, and will snap in half instead of bend.
Fun Fact
The first gold nugget found in North America was discovered in 1799 in North Carolina. It weighed 17 pounds, and was originally used as a doorstop before the finders identified it as more than just a rock.