The sizes are handled using talents with different degrees of impact (giantism or shrink)
The sizes larger than medium are costed at 10, 20 or 40 for Minor, Major, Greater - yielding Large, Huge, Gigantic
The sizes smaller than medium are costed at 10, 20, or 40 for Minor, Major, Greater - yielding Small, Tiny, Miniscule
With size changes come impacts to ST, QU, Stalking and Endurance.
ST and Endurance benefit from larger size, but QU and Stalking are penalized and for smaller sizes the reverse impact is realized.