My sister made a sleeping/TV/guest room from a space so tiny that we called the result "The Columbarium," and yet my 6"+ son was perfectly at home in it. Of course he now serves on a submarine, so I am not sure if he was warped before or after using the columbarium!
The space had been reclaimed from under a high and wide stairway leading to the upstairs of the house, a tall Queen Anne set on a very high foundation. My sister occupied the garden apartment (ground floor) and the stairs led to the landlady's house above. So the space was slanted, high where it joined her apartment just inside her front door, and about 24" tall at the very back. The floor space was about 5' wide by 7' deep. It was all finished out with drywall and painted white, and carpeted. The doorway into it was a beautiful bi-fold door, mirrored-- a closet door, actually, and people visiting never knew there was a "bedroom" there unless they were clued in. (I used to sleep in there when I visited and feel perfectly safe because if the bogeyman came into the place, he would never find me!) It had a tiny, functional, octagonal porthole window. There was room for a single-size futon bed on a frame about 12" off the floor, a narrow, low shelf under the window, duffle bag space at the foot of the bed, and, next to the entry into it, a recessed closet space with installed cabinet and clothes pole. Because the bed was so low and the rest of the furniture was smallscaled, it did not feel claustrophobic. There was plenty of room to stand in the front third of it and it felt quite roomy when one laid down or sat on the bed. She decorated the slanting ceiling with poster-type stuff. The whole thing was white, with accents in primary colors. There was a 10" TV in there, a boombox stereo, books, and so forth.
Another person I knew made a sleeping space out of a large walk-in closet. He was too broke to heat his whole house adequately, so he set up a double bed in that closet and heated it with a lightbulb. It was cozy. I believe there would have been room for some shelving over the head of the bed, and there may have been room to walk along the bed on one side.
We have a small (about 7' square) space at the top of our stairs, outsidie our bedroom and before the door that leads to the hall for the rest of the bedrooms. At one time I am sure it was used as a nursery, handy so parents could hear baby at night. I have laid out sleeping space there for visiting adults. It also has a chair and a long, low cabinet like a dresser. I had another landing space like that in another house and we made a sitting room out of it for sitting up to wake up in the AM and look out the window, or for reading late at night without going downstairs.
The trick to making small spaces work is getting multiple uses out of any furniture in the space. A captain's bed with storage drawers under it, or even a loft bed that lets you put a desk or chair or dressers below, can make a lot of storage with only a little footprint used up. Shelves running a foot below the ceiling, all the way around the room, can provide more storage without crowding the living part of the room.
I think one of your best sources for ideas, where you can see a range of solutions, would be to visit an RV sales outlet and cruise around the inside of various kinds of campers. Free gawking.
There is a US cable-TV show called This Small Space. Maybe you can be on the lookout for that. At our house we joke that one of these nights they will do a feature on custom-decorating your prison cell, and "personalizing" your space there.
I have heard that the trend "for the millenium" is moving away from large sprawling homes and back to small, functional, affordable housing beautifully trimmed out. (As the baby boom families become empty nesters, retirees are looming as a big market share on the horizon.)