attacking large targets

Somewhere in the back of my mind there is a footnote that some spells need a higher magnitude to affect big targets. But my search function didn't yield any results.

Let's take the two best known attack spells:
1.) Pilum of Fire
2.) Demon's Eternal Oblivion (the magic realm variety for this example)

What happens if I use those spells on a magical ice giant?
a) Nothing hapeens. You need to add sizes.
b) The spells work as normal.

If your answer is a: How many extra magnitudes are needed for which size?

Spells need a higher magnitude to affect a larger Target, i.e. the spell-parameter "Target". They don't need a higher magnitude to affect a larger target, i.e. the thing the spell targets (unless it is also the Target). So,

(A) Pilum of fire creates a fire of a certain size and intensity, and that is the Target of the spell. It therefore doesn't matter what the spell is aimed at, what is the target of the spell - in all cases the created Target is the same, and thus no extra magnitudes for size are needed.

(B) Demon's Eternal Oblivion is more iffy. On one hand it can affect the demon (or the Magic creature) as a spirit, and spirits don't have size so that no size modifier is required. On the other hand it can affect the demon as a creature, and then there is a size to the Target and hence size magnitudes may be needed. I'm not sure.

For DEO, it's Target is the might score, which isn't dependant on the size of the creature. Thus, no size is needed.

Now, if it was a perdo corpus, for example, and you were targeting a giant (human-ish enough to be covered by corpus), you'd need size modifiers appropriately.

I think a trickier one for people to spot is Wizard’s Icy Grip.

Perdo ignem, but you're affecting the target (small t) as the Target (big T), so therefore you do need size mods. That's my take on it.

That's mine, too.

So, when you do need extra magnitudes for a larger target, can they be added at casting time or does it need to be designed as a separate spell?

Using the current example, if I were to run up against Stellatus the Dragon (Size +8), could I cast my regular Wizard's Icy Grip on him as a level 45 spell or would I need to tell Stellatus to hold on for a season or two while I go back to my lab and invent Gigantic Icy Grip (PeIm 45, affects up to Size +10)?

Separate spell, unless you have Flexible Formulaic Magic (well, even that isn't enough for Size +8) or an appropriate MuVi spell. That's why the PeIg attacks aren't necessarily better than the CrIg attacks. They're better by ignoring armor but worse because the chill the Target who is the target, whereas the the CrIg attacks make Target fire on the target.

I think that's a bit nonsense, and rather it destroys the heat in a space that would be equal to a great hall's hearthfire or a large campfire. There would still be damage done to a large creature, and the damage increments are more than enough difference to make up for the fact that it may not be as much damage done, or that the creature isn't wholly within the space affected by the spell.
IF we are going to be pedantic about it, based on just the description of the spell, the spell should require Form requisites for affecting Targets of other Forms, too. But it's clear that the Target is the heat, and not any other Form, just like Pilum of Fire.

I think you meant to quote something else. Clearly there is disagreement here and so this spell must be tricky.

Except that if you look at the examples spells that do that they use T: Part specifically. So you could adjust to T: Part to chill the air to cause damage, which would add a magnitude and thus deal level vs. damage issue I mentioned when comparing CrIg to PeIg. Of course, the T: Part idea does allow Flexible Formulaic Magic to pull this off against other targets.

I don't think I did. I just checked and I'm sure I didn't.

Where does T:Part come from? The base size for Ignem is a large campfire or the hearthfire from a great hall. Is it so unreasonable that one would be able to remove the (or all) heat from a similar sized space? Ir is it because it might affect something (a small "t" target, or recipient) that is larger than the supposed size limitation of the Ignem Target size? What if, it created a bolt of super chilled air that was so cold that it caused frostburns that caused +20 damage? Which is really what we're talking about when it comes down to the mechanics as far as damage go.

So you're saying it's nonsensical that I say the spell is tricky to interpret in terms of target/Target while being in disagreement with the only two others to post opinions? Really? So why is there disagreement? I figured you meant to quote BlackLiger's interpretation and my agreement, saying you thought our interpretation was nonsensical.

T: Part is because you're not trying to chill all the air in the world, just a portion of it.

How about this? Let's say I want to chill a large (+2 magnitudes) region of air within R: Voice so that the cold fatigues anyone inside of it. Just what you're saying but to get a large region and possible a whole bunch of people inside of it. I would build it this way: Base 4, +2 Voice, +1 Part, +2 size. Do you disagree with this?

Also, have you looked at the PeIg guidelines? PeIg (for cold) chills objects/people. It doesn't create cold.