IIRC, one of the reasons 50L was chosen was because most of the cars in the competition had ~50L tanks. Choosing 25L would require a very complicated (and scientifically inaccurate) method of getting the same amount in each tank.
Last time, they used the EPA estimates and drove X kilometers to burn the little bit of extra fuel... but burning 25L of fuel that way would be wasteful and inaccurate at best.
40L might work...
But I think having a support car following with some jerry cans and repeating the 50L challenge would be best. That would allow for straight comparison with the old models, too.