Full body shrinkage (and pee)

Hands shaking with excitement, John decided to add more right away. He'd always wanted to get made fun of for being short, but he wasn't sure how short he wanted to go. He could go down to his sister's height at 5'6", but that would only be 2 inches shorter than he already was and he'd actually heard his sister get called tall from time to time. Eventually, he settled on 5'0" flat. Short enough to be humiliating, but not so much as to become too inconvenient. He could always change it later anyways.

"I'm exactly 5ft tall with a petite frame"

After lifting his pencil, he felt a sudden wave of nausea as his body suddenly felt like it was disappearing. As he regained his bearings, he realized he was on the floor with his shirt loosely draped over him. His pants were almost completely off, and there was a mysterious puddle underneath him as well. He realized his body literally just shrank rather than changing it so he'd always been this height. Suddenly more aware of his family's voices beyond his door, he panicked and wrote in the book.

"The changes and events from the past 10 minutes are normal"

As he lifted the pencil, he immediately felt the warm embrace of his new clothes. With a sigh of relief, that he didn't mess something up with the last sentence, John started to get back up and take in his surroundings.