Actually, cars ARE segmented by interior space. At least by the US Gov't. There are many car classification schemes around the world. The Chinese do use vehicle length, the Japanese use engine size and the Europeans don't have a formal classification scheme.
The US uses interior volume.
Vehicle size class - Wikipedia, the free encyclopedia
So while the length and price of the Model 3 will be similar to the entry-luxury compact sedans (3-series, A4, C-Class, ATS, etc), its interior space (passenger, trunk and frunk) will likely be a class larger...mid-size.
This is the big take-away from the size view: the passenger room is similar in length and height to the Model S. With the smaller overhangs, the cargo room is obviously smaller than the S.