Harry acted on pure instinct and rolled to the side with as much strength as he could muster. He felt if he stayed in place he would crash against Hermione's body and either die from the impact or get inevitably crushed later, so he moved with such energy he was able to escape just before the giant witch finished putting her panties on.
And now the boy who lived saw death approaching him from a different angle, freefalling from a height that was sure enough to kill him. This time his instinct was just making him flail his arms and legs pointlessly in a desperate trick to overcome the force of gravity, or to materialize a miracle. If he had his wand he could have probably used accio on something to try and soften his fall, but the broken half he found now was trapped in Hermione's panties.
Life flashed before Harry's eyes as he saw the floor getting closer and closer, a tragic life that still didn't reveal to him at what moment he became so little, or at what moment he ended up stuck with Hermione on her bed. But somehow whenever Hermione was close to him she managed to turn his fortune, and this time was no exception. All it took was a slight movement from the giantess, her right foot moving to scratch an itch on her left calf, and it provided the shrunken wizard a cushion. Even if she wasn't even noticing him, Hermione moved her foot at the right time in the right place, and Harry ended up hitting the soft part of her arch, rolling a bit before falling again, this time from a non lethal height until he finally hit the floor.
The whole experience that lasted mere seconds was so terrifying and painful that Harry could only cower in a fetal position in a futile attempt to protect his body, not caring about being close to the giant white pillar that was Hermione's left foot, or the tremor caused as she put her right foot back to the floor, or the gust of wind that moved his body slightly as Hermione's towering form moved away from him, apparently to put on her skirt.
It took Harry a couple of minutes to finally get up and try to think about what to do, accepting how as scary as being near Hermione was he definitely needed her. He was powerless without his wand, at the mercy of even small animals at this size, and his best choice was trying to contact her or sticking around with her until he saw another friendly face that could help. Right now she was alone in the dormitory, and was finishing putting off her socks so Harry knew he didn't have much time. He summoned the remaining energy on his body and started running towards his friend, hoping to be able to climb her sock and up her robe, or at least hitch a ride on her shoes.
What happens next?