A half bath needn't be large. You will have enough space if you can find a spot in your house that's about 3 to 4 feet wide and 6 to 8 feet long. If it's any smaller, it will be uncomfortable for people to access. A larger half bath is unnecessary, and it's not likely that you'd find that much "extra" unused space in your house.
Shown: This parlor bath in New Jersey is able to capture a hotel spa look in a limited space of 40 square feet.