Armor does not resize to fit the wearer, as stated in MiC it, weapons and I think shields, do not resize. I know weapons and armor do not for sure.
Sizing is Weapons only.
Alternate armor sizes, etc is before enchanting cost. But you do multiply the material costs. (Adamantine, Mithral, etc)
Thus, large Mithral fullplate is 21,000. And for +5, 46k.