What does # cu. ft. even mean?

[BobVosh]

Thing is, while "size" works great with things like, I dunno, an ogre, it's less fantastic with things like water. How much is a "Huge" or "Large" amount of water, as opposed to "small"?

Find the weight of the large, huge, and small water elementals. Find out how many gallons of water it takes to equal out these weights. Wa-la!

Probably the easiest way to do this is off weight rather than size anyway. So you can reduce 20 lbs. The object was 40 lbs. It is now half as large. Simplistic.