Maybe the shrunken man is trying to find a way to return to his normal size while in the grandmother's house. He might face various challenges like avoiding being stepped on or finding a magical item to reverse his condition.
The shrunken man is the central figure as the story likely revolves around his experiences in the giantess grandmother's house. The giantess grandmother is also key as her actions and the nature of her house play a big role. There could also be other characters like the grandmother's pet, which could be a threat to the shrunken man. For instance, if it's a big dog, it might sniff out the shrunken man and cause trouble for him.
Perhaps it's a stranger who was cursed and ended up in the grandmother's house in his shrunken state. He then has to deal with the grandmother and the strange environment while trying to break the curse.
It could be that the shrunken man is on a quest to find a way to return to his normal size while in his grandmother's house. He might encounter various obstacles like tiny creatures that try to stop him, or he has to solve puzzles to find a magical solution hidden somewhere in the house.
Obviously, there's the shrunken man. And of course, his grandmother might be a character, even if she's not present physically, her presence could be felt through the house and the things in it.
Well, it could be that the shrunken man was shrunk by some kind of magic or accident. In his grandmother's house, he might encounter miniature versions of things he thought he knew. There could be tiny fairies or magical creatures that only show up at his size. He might also have to find a way to get back to his normal size, and in the process, he learns more about his grandmother's past and the history of the house.
One interesting story could be about a shrunken man who finds himself in a giantess's garden. He has to navigate through the huge plants and avoid being stepped on. The giantess, not knowing he's there at first, goes about her daily activities like watering the plants which becomes a huge obstacle for the tiny man.
Perhaps in the story, the shrunken man is a scientist who was working on a theory about alternate dimensions. His experiment goes wrong and he ends up in a dimension where there are giantesses. The giantess he meets is curious about him. He has to communicate with her, despite the huge size difference. They develop an unlikely friendship as he tries to figure out how to use the resources in this new dimension to get back to his own world and his normal size.
The main characters would obviously be the shrunken man and the giantess. The shrunken man could be an ordinary person, like a school teacher or a mechanic, who suddenly finds himself in this strange situation. The giantess could be a powerful being, perhaps a queen in her own world, with a kind heart despite her intimidating size.
Some stories end with the shrunken man accepting his new life with the giantess. He may build a new community or life within the world of the giantesses. He might even fall in love with the giantess who has been protecting him all along. In this case, they live happily ever after in their own unique way.