You need a number of ranks equal to the PP cost of the spell being cast, regardless of what is increasing the PPs.
For example, a Mage with an Elemental Bolt spell would need 4 ranks to cast the base spell, if he was not scaling or wearing armor. If he decided to scale it to a Small attack, he would need 6 ranks in the spell since it cost 6 PP to cost (4 base + 2 scaling).
Then if he threw on a suit of soft leather if he cast the spell at Tiny, he would need 6 ranks in the spell (4 base + 2 PP cost for the armor), 8 ranks if he scaled it to Small, etc...
Then for each PP beyond the base cost he is taking a -5 to the roll as well, and the casting time extends, etc.