If a small person fell and cut the whole area of their knee and a larger person fell and equally cut the same area of their knee would their pain be greater?

Not necessarily - Pain is completely and totally subjective, meaning each person experiences pain differently. Part of it has to do with the person's experience with pain as they age, any pain drugs/anesthesia they may have been given during and after surgery when they were small children (tonsils, etc), all those and many other factors alter each persons sensitivity, or Tolerance, to pain.

For example, as a chronic pain patient I've experienced extreme pain every day for the past 12 years; 11 of those have required the use of opiates to maintain a fairly normal lifestyle. As such, my pain tolerance is extremely high. So what might be excruciating for you or someone else might feel like hardly anything to me.

In your comparison, the opposite could be true - the smaller person could have more pain than the bigger person, if you consider the smaller person a child and the larger one an adult who has had more pain experience, while the child has had little or no serious pain experience.