Scaling up water for cooking soup
1.5 litres for 4 servings is 375ml per serving (plus some volume from the veg which I'll ignore) assuming no water boils off. That's a sensible portion. I reckon my soup bowls hold just a little less than that, but you'll leave some in the pan when serving . So I doubt you lose a lot of water when you normally cook it. 

That said, I'd err on the side of caution and (i)  measure the bowls (both the normal ones and the ones you plan to serve it in if they're not the same); and (ii)  start with a little less water than you think you need - adding water is quick, boiling it off less so and you don't need the hassle of your soup taking too long.