I think there's a part that says "Unless you have a better idea, armor is XD10 where X is the size of the creation."
Failing that, you could just say so anyways. So a 2-inch minivan has 2D10 armor, and a 2-inch tnak is foritified, giving it 3D10 armor.
Or even do the following if you have more dice than sense:
Creation Armor (x=size of creation)
xd10-2 for [relatively] light armor (skin, fiberglass, etc)
xd10 for [relatively] medium armor (tanks, mecha, etc)
xd10+2 for [relatively] heavy armor (ramming vehicles, foritified walls, etc)
If you want, of course.
It's a secret to everybody.