While outdoor spaces are technically not rooms, the size requirements are similar. Much like a dining room or a family room, you will need enough space for your furniture and to comfortably circulate. Measure the spaces in your house to determine if a similar layout will work for your outdoor deck, porch or patio.
Also be sure that your outdoor space is in scale to the rest of your house. A gigantic outdoor space next to a small home or a very tiny space next to a large home could easily feel out of proportion and awkward. A good guideline is to make your outdoor space comparable to the rooms inside your house.