Answer:
0.5 liters
Explanation:
If they were to get 0.25 then they would still be missing some more for the recipe. If they get 0.5 then they would have just a tad bit leftover. Less than 0.027 liters. Which is the smallest amount that they can have left over. 1liter and 1.5liters are over twice the amount of the recipe. So that wouldn't be a wise choice unless they wanted to make double or triple the recipe.