You came there probably wondering how many squares to use in game to replicate the size of a real house…
Based on furniture size, one square (unit) in all The Sims games seems to be 3 feet (0.91 meters), or 9 square foot.
But this is not important, you can consider each game square to be 1 square meter (= 10.7 square foot).
However, if you want to replicate real houses, you should NOT pay attention to matching size of real house, instead you should adjust the house size to make it suitable for your Sims family size.
In Europe a 3-bedroom house for a family of 4 is in average 100 square meters, but in The Sims you need about 150 squares for a family of 4. You need extra space for things that in real life do you not need, for example chess table. For a double bed you need a room of at least 4×4 squares, but in real life bedrooms can be smaller than 3.6 x 3.6 m. A two-car garage is 6×6 meters in real life but in The Sims 2 you need 10×8 squares.
The corridors in real life are 90 cm – 1.2 m wide depending by each country’s building code, but in The Sims you should make them 2 square wide to prevent Sims getting stuck if 2 Sims walk in opposite directions.
Actually corridors just make Sims to waste time going from one task to another. For maximum playability you should reduce the number of rooms to minimum possible and avoid hallways. A house is playable even if have only 2 rooms, one for noisy things (TV, stereo, computer, piano, etc) and one for beds, example: The Cube of Glass or you may want to find a compromise between realism and playability, for example Tiny House IV.
Look at the real-life buildings designed by me in The Sims… how I adjust the size to game.
For example Palace of the Parliament, a 12-story, 86 meters tall, 270×240 meters building, designed in game as 6-story 48×42 squares The building size is reduced but is still realistic because the length / width / height proportions are kept.
Or, Home Alone house which was 21 meters long I designed it slightly smaller, 20 squares long, and removed few rooms because real house have too many rooms for game needs.
In case of average houses you need to design them in game bigger than the size resulted from direct conversion of meters / feet to squares.