Vehicle Size- Does size matter?

[EOT]

What determines a vehicles size?
1) Weight
2) Length
3) Height
4) Width
5) Class
6) Barrel or weapon size
7) Crew size
8) Something else?

Vehicle sizes range from 2 to 6
Example: Tank Transporter 4 / Tanks 4or5 (Tank Transporter should be 6 or 7)

Most OOB's have many size discrepancies between vehicles and countries with same equipment.

Is their any way of getting a set of rules as a guideline for vehicle size?

This issue is critical to fair game play as size affects spotting and hitting.