At maximum, consider the case where the fuel was in such a volume but scattered elastically at the moment of impact. The ground does not recoil, so the fuel can conceivably spread at the aircraft's former speed, but fanning in (potentially) all directions. The speed of the aircraft has already been presented to you from the FDR. For sake of argument, assume one second between impact and complete engulfment of the fuel (this time is an estimate, but there is absolutely no way to predict just how long it took to ignite, or where). Multiply the speed by one second, and add it to each extreme of the first calculation.
Show us your answers, and we will see if they were computed properly. Expect answers for the first on the order of 10-30 meters, and > 300 meters for the second.