On a tangent: Assuming the first line is "def largest_square_plot(width, height):", you may be interested to know that your code computes the greatest common divisor [1].

If I'd been the one to write this function, I would have done it like:

  def largest_square_plot(width, height):
    """Computes the largest square to tile a plot of the given width and height."""

    # because we want the grid of squares to fit exactly
    # the size of the squares needs to divide both width and height
    # to get the largest square, we use the greatest common divisor
    from math import gcd
    square_size = gcd(width, height)
    return (square_size, square_size)

... but now I realize that probably not everyone is intuitively familiar with the kind of math that involves the GCD, so your code is actually much more intuitive without that background.

[1] https://en.wikipedia.org/wiki/Greatest_common_divisor