The main character's sister writes a novel because she received some kind of message telling her what is going to happen in two years; she had to let the world know, but in an indirect way.
Everyone becomes ~2 cm tall. In return for their height loss, everyone gained an ability. The main character had the title "Prince" and a barrier ability. That'ss basically all I remember.
I think his sister was also trapped by a person with the ability to enlarge themselves.