Okay, so there has always been one thing about all of the rulebook editions (since 2001 anyway) that confused me. I know this has probably been stated a thousand times before but, when it comes to size damage, do you use the vehicle's size in inches as its SP or the number of its structure level? For Example:

Mk II(C) Typhoon Main Battle Tank

CP: 52

Move: 8"

Armor: 3d10

Size: 69"

Size Rating: 3

Crew: 1

Passengers: 2

.50 caliber MG: CP: 69 UR: 4 RNG: 10” DMG: 2d6

100mm HVG: CP: 12 UR: 8 RNG: 24” DMG: 4d10 exp

2 Mk II ATGM Launcher: CP: 69 each UR: 4 RNG: 12” DMG: 2d10 exp

Now suppose that my mighty tank just got hit by an ATGM, and its armor roll wasn't so mighty, which cost it two structure points in damage. Would I subtract those two from the vehicle's size (69"), causing it to lose the number of weapons it could use in one turn, or would they subtract from its size rating, leaving it only a size 1 build in terrible shape?

Basically, my question is which one do you guys use more often? If damage subtracts from its overall size in inches, that means your vehicles have a much better survival rate unless their hit by like a nuke or something. If the damage is taken from the size rating then it is a lot more realistic as even the most heavily armored vehicles can be destroyed with just a few rockets on a good hit.

Thoughts?