I don't have a tiny house, but my private room listing is quite small in terms of square footage. However, it doesn't feel cramped because it has lots of windows (so many that there is hardly space for wall art- the nice views are the art) and has a high ceiling.
I also didn't try to cram too much furniture in, or try to host more than one guest at a time, as only a single bed fits in while still giving ample space to move around. The closet and desk/countertop are built-ins, so the only actual furniture are the bed, a chair, and the 2 little nesting teak bedside tables on one side of the bed.
There are no knicknacks or purely decorative items, either. Everything has a purpose. It's important to keep small spaces uncluttered.