In order to estimate bite force with any reasonable confidence, we'd need to have a rough idea of how much jaw muscles Spinosaurus had. Unfortunately there are no good cranial materials to reconstruct jaw muscles in Spinosaurus - the bits at the back of the skull where the muscles would have attached are not known for Spinosaurus. Therefore we won't know for sure.
However, we can fairly confidently assume that Spinosaurus would have similar skull proportions to those of close relatives like Baryonyx or Irritator. These theropods had long narrow skulls with not much space for jaw muscles. From what we know of Spinosaurus skull materials, we can be sure that it also had smallish jaw muscles (for an animal of that size).
Extrapolating from size estimates (I presume body size, e.g. body mass, body length, whatever) would not give you a good estimate for Spinosaurus, for the very reasons I outlined above, i.e., spinosaurs had smaller jaw muscles compared to other theropods of similar sizes. Using body size will grossly overestimate bite force.