I'm thinking of making a giant witchfire (that formed from an annis hag) for my campaign.
Giant template increases the creature's Strength and Constitution. Undead don't have the Constitution score though, and use Charisma instead. Incorporeal creatures don't even have Strength score. Should I increase its Charisma instead of Constituation?
Giant template increases natural armor. Incorporeal creatures don't have natural armor. Should I give it a different type of armor instead?
Giant template increases the damage dice by 1 step. Although listed in attacks, witchfire's incorporeal touch and witchfire bolt feel more like supernatural abil;ites. SHould its damage be increased?
Witrchfire can use disguise self at will. Can it use it to hide its flames?
Giant template on incorporeal undead is really bad. It does nothing.
If I were going to adjust it to something effective I would probably just apply untyped increase where I would expect them to be - +1 AC, +1 to hit, +HD*2 hp etc. It is unlikely that the type on a critter will ever be relevant.Be aware that because it is incorporeal any bonuses may have a disproportionate impact. Be aware you may have to fudge it a little in play.
I would not increase the damage.
Disguise self probably shouldn't hide the flames, but there is also nothing to say it can't suppress the flames either. As we are ad hoc ruling anyway I would probably just say they can suppress the flames whilst under the effect of disguise self.
Giant template still increases reach for those touch attacks. In this case, though, that doesn't matter much. If you apply the giant template, I wouldn't bother increasing the CR at all--without natural armr, weapon attacks, Strength or Constitution, it's a mostly cosmetic change.
Even invisibility wouldn't hide the flames (well, technically it wouldn't hide the light given off by the flames). I don't think disguise self would be sufficient. It's ultimately up to you, though.