Litres (though grams is possible too)
Put it in a bathtub of water and measure the rise in water level. Multiply by the length and breadth of the water.
Displacement is the amount of space that an object takes up when placed in a fluid. Have you ever noticed how the water level rises in a bathtub when you get into it? The amount of water you are displacing is the volume of your body that is in the water. so by measuring the displacement of an object, you can measure the volume of the object.
The simplest metod is to use a calibrated bucket.Frequently the volume is 15-20 L for a bucket.
That amount and more
The common metric unit of volume is the liter, which would be appropriate to measure this amount of water. The SI unit of volume is the cubic meter, though.