You need to describe your problem in more detail e.g. what exactly do you mean by "track steps of people on a marble stair"?
Lets say your Force Sensitive Resitors were big enough what would be your plan - to put one FSR on each step?
Is there no carpet, what about the wires.
Are you trying to 'see' each step a person takes, what happens if there are several people on the stairs at one time?