You haven't given any information at all to describe the rectangle ... perimeter, dimensions, etc. That's kind of important.
Using all of the information you provided, I can only tell you this:
-- The greatest possible area = (1/4 of the rectangle's perimeter)² .
-- The least possible area = the smallest positive number you can think of.
-- Their difference is
(1/4 of the perimeter)² - (the smallest positive number you can think of) .
