Magic armor adds an enhancement bonus to AC, so a set of +5 black iron dragonscale adds a total of 15 to the wearer’s Armor Class (10 from the scale armor and 5 from the enhancement bonus). If you’re not proficient with the armor type, you take –2 penalty to attack rolls and to your Reflex defense but still gain the enhancement bonus of the magic armor.
The category determines what kind of armors can be enchanted with that particular set of qualities. “Any” includes all armors: cloth, leather, hide, chain, scale, and plate. You can resize magic armor with the Enchant Magic Item ritual.