From MangaHelpers:

Natsumi is in the same high school class as her childhood friend, Haruyuki, but he has been treating her coldly and formally. One day when she wakes up, she suddenly finds that a doll-sized version of Haruyuki has appeared in her room! It seems that no one else can see or hear the tiny Haruyuki, and when she goes to school she finds that the original Haruyuki still exists. Where did the tiny Haruyuki come from, and is there any way to get him back to being a part of the original boy? [tethysdust]