Seats five is interesting... but I'm not quite seeing the perspective. Its hard to tell the size difference.
From my eyes, it looks like its roughly the size of a Scion, maybe only a little bit larger than a Honda Fit. Which seats 5... technically... but its not too comfortable. Its more of a 4-seater with a 5th seat that may be used if you care about it.
Because of the lack of combustion engine, gear box and drive train the interior of the Model S is huge compared to other similar sized conventional cars.
I imagine the model 3 has the same benefits and the interior is a lot larger than it looks.
From my eyes, it looks like its roughly the size of a Scion, maybe only a little bit larger than a Honda Fit. Which seats 5... technically... but its not too comfortable. Its more of a 4-seater with a 5th seat that may be used if you care about it.