Small rooms will feel cluttered if you fill them with lots of little plants. If you have lots of little plants in a little room, group them all in one location. Put in a plant rack or find a table top to gather them all upon. Another way to remove the cluttered feeling is to use the same pots for all the small plants in one room.
One medium to large plant in a small room can be very dramatic and change the whole way the room feels. A large hanging plant, or a tall plant with the pot seated behind a chair won’t take up much space.
Put large plants in large rooms otherwise they will get lost. If you have some smaller plants you wish to display in a large room, get a pot large enough for all of them and put them all in one pot. Long, shallow pots work best for this. Or place the smaller pots inside a large pot and place some moss on top to cover up the little pots. This works really well with orchid plants.
If you have a large plant in a room think about planting some smaller plants in the pot with it, or adding smaller containers of plants around the large plant. Outside you often plant hostas or something around tree bases, try for the same effect here.
For more information:
Peaceful Garden: Bathroom Plants