Common coins are nearly identical in size and weight, regardless of metal. Unless you, the DN, decide otherwise for a particular realm or culture, assume 100 coins weigh 1 pound.
A warrior dragging home thousands of coins from a dragon's hoard quickly learns that treasure can weigh more than their armor. For this reason, seasoned adventurers and merchants often convert bulk coin into gems, stamped trade bars, etc. These hold immense value with a fraction of the weight - far more practical for those who travel light or deal in fortunes.