The capacity of a barrel is a measure of volume. The SI unit for volume is the cubic metre. A barrel with a volume of one cubic metre would a big barrel. It could be better to measure it in thousandths oh a cubic metre, a thousandth of a cubic metre is called a litre.
milliliters
metric unit
meters
mL.
The mass of any coin is typically measured in grams; the capacity or volume would be measured in millilitres.
milliliters
Milliliters is the metric unit to use for the capacity of a thimble.
metric unit
It would be millimetres.
meters
liters
liter
You would use millimeters for the measure of the capacity of a paper cup.
Ounces
mL.
Liters would be easiest to use.
Probably none because a stream does not have a capacity but a rate of flow.