The term mansion is somewhat flexible. It may relate specifically to size or might encompass the idea of certain amenities that could be possessed by those who are very wealthy. In modern real estate sales, it may also relate to specific square footage of a home that is luxuriously designed.
Early use of the word doesn’t always imply a huge home. The word manse, which is identified as Scottish in origin, was used in England and Scotland to describe homes that were held by clergyman. Even farther back, mansion is related to a Latin term that describes the act of staying someplace or remaining somewhere. This definition is not that far off from the term as it is understood now, and staying in a large and elegant home is clearly attractive.
It might simplify matters to describe the mansion as a very large house. In Europe, and certainly in England, such a house would have been able to accommodate multiple servants and very likely had facilities like ballrooms that were large enough to entertain very big crowds. Given the propensity of the upper classes to visit and socialize, plenty of extra bedrooms needed to be available too. These houses could be part of an estate, and therefore would be associated with one building devoted to worship, at least a chapel, and often houses or a small village. Alternately, as became more common, the home was built alone, usually on ample grounds.
In the United States, as the country developed, many types of large homes were built. These were usually not part of estates, but they could be impressive in size. They trended toward being somewhat smaller than typical European homes and, gradually, a definition as relates to square feet evolved. Most often, any home that is 6,000 square feet (about 557.41 square meters) or more, and that has various elements associated with wealth, like pleasure grounds, home theaters, bowling alleys or extensive adult recreation rooms, would be called a mansion. Until recently, average square foot home size of new homes built in the US was about 2,500 square feet (about 232.25 square meters), and so the size of most of these houses would accommodate two fairly large homes, if not more.
The US also started the trend of building the mini-mansion or McMansion, which typically is a step down in size from the traditional definition and is at least 4,000 square feet (about 371.61 square meters). These types of houses have become less popular, however, as the housing market struggled in the early 21st century. It also becomes more expensive to provide temperature and electricity to larger areas, which can discourage people from buying McMansions in uncertain economic times.