Harry's original height was 7'8, but he was shrinking 1 foot at a time, so he was gonna lose 1 foot of height, day by day.
He stepped out of his boots and stood against a door, using a book to mark his height. With a tape measure he moved towards the mark.
68, 69, 70, 71, 72...72!
He was now 6'8. He had lost an entire foot of height and the rest of his body had scaled itself down in proportion.
"Holy cow!" Harry said. "The fountain of size worked after all. I'm shrinking."
Indeed.
Harry was getting smaller.
and his clothes were getting looser.
and he saw his house seemed to be getting bigger. [in reality, Harry was shrinking.]