Size is a measure of the approximate volume and mass of Cybertronians.
Size has a derived Stat: Size-
Size- is not a penalty, it is a bonus used when being smaller is better.
Size- is equal to 5 minus Size.
For Massive characters, whose Size is greater than five, their Size- will be a negative number.

Size examples (approximate height, assuming humanoid-like proportions):

0: Humanoids
1: 8 ft - Rewind
2: 10 ft - Bumblebee
3: 12 ft - Jazz
4: 15 ft - Ironhide
5: 20ft - Optimus Prime