I have 2 questions here: 1) after I have used the spell mini how do I..

1.)You just use the Mini spell again to revert to normal size.
2.)If I can remember when I last fought that rat, I mainly used Refia(Or whatever you named her) for mainly using Curing White Magic spells, Luneth(Or whatever you named him) for offense with sword combos, Arc and Ingus(Or whatever you named them) for either Black Magic, offense, White Magic, or defense.

It's been a long time since I played this boss, so I can't remember everything I did, but I hope this helps you out.

What i did was have evryone red or black mage except for refia who was a white mage. dont bother equipping weapons just give them magic and armor.
rest and buy potions save outside the temple and collect all of the treasures exit rest and save outside the temple. then just get the rat make sure evryone is fully healed and use magic. on the first turn get white mage to use antartic wind then just use potion and cure. when you get out talk to the nepto dragon statue exit save rest and talk to the leader of vikings.