There is almost no difference between 15L and 20L unless you are using a TON of it in your recipe (and you shouldn't). You probably won't be able to tell the difference in the finished product regardless of which one you use.
if the recipe says 25, and my lhbs sells 20, 40, etc, i get the 20. if it says c110, and i can only get 120, that's fine too. 15 and 20 are close enough