Coins, Change and Weight

Coinage

RAW: Coins are generally 50 to the pound regardless of composition (copper, silver, gold). Volume is not generally stated but they'd be fairly small.

Making Change

As DM I don't find it terribly entertaining to make people track what kind of individual coins they are carrying. As a result, until and unless it becomes ridiculous it can be readily assumed that the party can make whatever change is necessary to properly split whatever cash they find or have on hand.

Weight of Wealth

The weight of coins can be ignored until it becomes ridiculous. Character sheets have been modified to ignore coin weight.