Tank volume refers to the volume of water in litres. Tank size refers to the dimensions of the tank (length, depth, height). Volume is good for quick reference, but size is also important because you could have a tank that is e.g. [1000mm long x 500mm high x 400mm wide], when compared to a tank that is [450mm long, 450mm wide, and 1000mm high] (column tank). They would both be the about same volume - around 200 litres, but the second tank would severely restrict your stocking plan. That's why it's important to include both volume and tank size when posting stocking questions.
Tank volume (in milli-litres) can be calculated by multiplying the three dimensions in centi-metres. To get to cm from mm you simply take the decimal place back 1 space (so 1000.0 mm = 100.00 cm, or 10.00 mm = 1.00 cm)
100cm x 40cm x 50cm = 200,000 ml
There are 1000ml in a litre, so 200,000 ml is 200 litres.
Hope that makes sense, sorry that was the easiest way I could think of explaining it. Using deci-metres (dm) makes it a straightforward equation but a decimetre (10 cm) isn't commonly used.
EDIT: NAyfee beat me to it and explained it a lot simpler lol.
Yeah, 379 litres is right (although not sure why it isn't rounded up to 400 litres) and the tank size should be something like 160cm x 50cm x 50cm I would have thought. Looking at all of the marine caresheets the tank sizes all seem to be in gallons rather than in feet or cm. Needs looking at I think.
Yes, like the column tanks I referred to, which are usually seen in hotel lobbies or restaurants, but I have seen a couple of them used in people's living rooms. Fish swim left and right more than they swim up and down, so horizontal space is much more important. A column tank would restrict you with numbers, but more imprtantly what type of fish you could keep. For example, a tang wouldn't have anywhere near enough swimming space in a 0.6m w x 0.6m l x 2m height tank, but if you turned the tank on its side it would be plenty.