Ice giant would be better.
So you guys are advocating for me transforming into an ice giant instead of the cold immunity? MuCo(Aq)30 effect for a size +3 creature?
Xavi
Not necessarily. I was actually just thinking of it as something else you have in your repertoire, either way is cool.
[edited to add:] actually with 30 levels at your disposal your ice giant form would be much larger than +3 size. Since giants are humanoid and land animals you would have a base 10 start, Range:Personal (0), Duration: Diameter (for sake of argument and economics) (+1), with 3 mags left over for size for a whopping +9 (elder dragon sized).
Well, since it would need some "kewl powerz" added to the giant to be a "proper" frost giant, I think size +3 or +4 should suffice and then add something like cold resistance or a chilly touch on top of that.
Cheers,
Xavi
Cold immunity effect sounds way more fun i would say.
With a size 9 Ice Giant, I don't think you would need any extra inherent powers (considering you would still be a full Hermetic Magus capable of casting spells without penalty (and at a helluva increased sight and voice Ranges to boot). Imagine how much physical damage such a humongous size would permit you to do with fists (or feet alone) let alone the hardness of the ice.
15-20 years out of gauntlet you can always just have this as a formulaic spell for whenever the need arises or the whim takes hold.
Certainly more versatile, I would agree. It also permeates the theme of his character entirely.
I love big targets.
You can see them before they can see you, and they are hard to miss.
Perhaps, but then big targets require added mags to any attack spell for size in order to have any effect.
Size 9 would just laugh at a typical by-the-book PoF. It would take a level 35 PoF and that is not easily sponted even for an archmage.
Size can have its advantages! 
I was more thinking of bows and aimed spells, but I checked and didn't find that size influences targeting in any way, so you are right.
It also says that size is tenfold for every +3 increase in size (page 18 and 192 ArM5).
So if a human is 5 ft (152 cm) tall - that becomes 50 feet (15,2m) at size 3, 500ft (152m) at size 6, and 5000ft (1,52km) at size 9?????
A human that is one mile tall must be really easy to hit,
her soles are about 250m (820 ft) by 100m (328ft). That is one nice trample attack!!!
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Ah, i think you got that "slightly" wrong... "Size" doesnt just cover ONE dimension you see?
Its like saying that a 1 cubic meter times ten becomes 1000 cubic meters because you just increased the lenght of its sides by 10 times but let it stay the same shape...
Only attack spells whose target is the actual giant would need extra mags for size. The target of PoF is not the giant, it’s the fire that’s created - this is supported by the Ignem guidelines, which describe base Individuals not as a size of target to be affected (as PeCo would) but as a size of fire to be created. Creating fire in a giant’s face still hurts, even if it hurts him less than it hurts a human (represented by his larger wound increments and greater Soak). Adding magnitudes for size would make a bigger fire, which might better be able to hurt the giant, but the giant is not suddenly immune to smaller fires just because they're small.
This is a common error which the Ars writers specifically addressed in the guidebox found on page 113 (Targets and Sizes). A one mag size increase for a human would create a giant 15 feet tall, not 50-60. A size 9 dragon would be comparable to largest known modern day whale, or probably in the vicinity of about 75-90 feet long from snout to tail (not including wingspan), but the weight increase would be into the 10's of tonnes.
You know, I am starting to see a size +4 flambeau magus with a HUGE ice sword in his hand cutting and WRESTLING large supernatural creatures instead of casting spells at them. Why levitate a boulder when you can simply grab it and smash it in the enemy's head?
Talk about weird uses of the school of Ramius, Boreas ansd Sebastian combined 
Now, I only need size +4 (or whatever size) arms and armour.... 
So, what size would a 9-15 metres high giant? I always thought "COOL!!!" Looking at the images of the magical giant in LotN.
Xavi
Let's see. +7 gives you 6 times as big as usual in every dimension, so a 10-12 meters tall giant.
Your size +4 flambeau would merely be four meters eighty or so - ogre/troll-sized
If you want to be a mile tall, like a mythical frost giant warring against the gods of valhalla, that's around +27, as far as I can tell.
ok. Will settle for +6 size, then
Cheers,
Xavi
+4 is the size of an elephant in the chart on page 192. I'm not sure how you arrived at +7 = 6x in every dimensions (and lets not forget weight, which isnt a dimension) but once you're past +5-6 its anyone's guess what you would compare size to in modern terms.
Mark Shirley did a chart that goes up to size +6 and compares +6 to "whale" (which I would argue is too small for whale sized unless it was a small minke whale perhaps).
On pure guesstimate work I would opine that +6 would probably be somewhere in the vicinity of 10 meters tall or 32+ feet tall and you'd probably weigh close to 20,000 kilos (40,000+ pounds).
Frankly what need does a giant made of solid ice at such height and weight need with armour??? Your ice body would be its own natural armour.
Let's see if I made this clear.... (formulaic spell there we go...)
Child of Ymir (MuCo(Aq)30)
R: Per, D:Sun, T:Ind
Transforms the target into a size +6 giant for Sun duration. This makes you around 12 metres tall and provides an increase in the damage increments.
(Base 10, +2 sun, +2 size)
Any other changes this increase in size would introduce? The book of beasts mentions that each point of extra size makes a beast gain +2 strength and lose -1 quickness. So a +6 size giant would be at +12 strength over my base strength and -5 quickness (but a much increased stride length so it can still outrun a human!! Sounds correct or would those modifiers require extra magnitudes, so my magus would be a str +1 (his human strength) giant? Sounds quite weak, doesn't it? +13 strength (and -6 quickness sounds better, doesn't it?
Damage increments being 1-11, 12-22, 23-33, 34-44, 45-55.
I am unsure how to determine potential increments in soak there....
In case you wonder what vision got struck in my mind, Look at Lion of the North page 125.
I can see my mage having s series of spells for size +3 to +6 and selecting the most "honorable" one to put him on par with the beast he is hunting down. Specialization in smashing weapons: select a tree of the appropriate size, make it tough with magic and use it as a large mace. Brawling/engaging the foes in physical combat sounds like what a reckless giant descendant would do when it comes to hunt dangerous magical prey.
Cheers,
Xavi
Not sure if I would go with Sun Duration. Why stay that way all day when you really only need it for battles per se. Diameter would save you a mag which you could use for an added effect. Also, I see no Aq req in that spell so you arent making an Ice Giant I guess?
As for strength, yes I would say that the bigger you get the stronger so a 12 meter Ice giant would probably be +12-13 strength as you say (which might be weak by giant standards, don't know never met one). 
Well check out Stellatus' soak at +8 size and extrapolate downwards perhaps?
Hmmm you best not make a habit of tearing up the wilderness and slaughtering animals or we might come to blows quite literally
More a frost giant than a giant MADE of ice. I can make that, though. It is only aesthetics, so it would be an Aq requisite without a magnitude increwase after all.
Added it to the above spell
The damage increments are made using stellatus as a benchmark, indeed 
Only SOAK remains to be seen how it is determined. An ice giant with a +2 soak sounds rather ridiculous, doesn't it?
Str +13,
Stamina..... --> and soak
Dexterity +1
Quickness -7
So quite heavy duty: slow moving but it hits hard. Add a magical ice sword the size of a tree and there we go with a cool ice hunter idea.
Xavi
Well I would only be guessing on soak but my thought would be to take the highest normal human sized armour soak (+9 I believe) and add +1 for each size above 0. So +15 soak sounds about right (perhaps even low by giant standards, but then ice can chip. There's just so much of it (you) to chip, a few gouges would only translate into flesh wounds when you reverted back to human form.
Stamina I can't say but why not use the same formula as for strength unless it is explicitly written somewhere?