OK, good to hear about the learning as well as making the Vs and Fs match, I'll get on that when I get home. Is there a final definitive answer on the Might though? Is it considered 10 or 8? That's where I'm getting different answers.
I would go at it like this.
Start with a base of 10 (companion level).
Add the creatures natural size.
Abnormal size is figured in by taking a specific Virtue, Flaw, Quality, or Inferiority. Large, Small Frame, Gigantic, Miniature. This ajustment would not affect might.
Example - Cidito.
Base 10, minus -3 for natural cat size equals 13. The Large Virtue gives him a +1 to size, resulting in a -2. But this does not alter his might score.
Example - Donnar
Base 0, might 10. Gigantic is purchased as a Major Quality, so this does not change might and his size is +2 (bigger than Marmaduke, smaller than Clifford), as large as a horse.
See that's what i thought, but I've been getting different answers. In the book if you change the size without buying the quality you change the Might. So it seems I did do it right by going size 0, Might 10 and then buying the Gigantic Qaulity. i still have to balance the virtues and flaws and maybe re-look at his Qualities.
Alright done a little re-working of him. I added Qualities from the Bjornaer book which saved me on some Virtues and I moved things around, though one of the qualities cost me the Survival skill. I think this adds up now, but suggestions are welcome.
Donner the Giant’s Dog Familiar to Alexei
Might: 10 Season: Summer
Size: +2
Str +1/+5
Sta +2
Int -1
Per +4
Pre -2
Comm 0
Dex +1
Quick +2/0
Personality Traits: Hound +3, Curious +2, Playful +2, Loyal +3, Brave +3, Greedy +2
Virtues: Enduring Constitution, Puissant Brawl, Tough, Unaffected by the Gift
Flaws: Greedy (minor), Magical Friend, Uncontrollable Strength, Warped Senses (Hearing)
Magic Qualities: Gigantic (Size +2), Personal Power (1), Personal Power x2 (2), Lesser Power (3), Improved Damage x2 (Bite), Improved Attack (Bite), Improved Soak x3 (Tough Hide)
Magic Inferiorities: Susceptible to Deprivation, Minor Flaw (Weakness, playing), Reduced Abilities
Qualities: Domesticated, Keen Sense of Smell, Sharp Ears, Thick Fur
Abilities: Athletics 5 (Distance Running), Awareness 4 (Keeping Watch), Brawl 5 (Bite), Hunt 4 (Tracking by Scent), Leadership 2 (Other Dogs), Swimming 3 (Rivers), Finesse 2 (Teleporting)
Combat:
Bite: Init +2/0, Attack +14, Defense +11/+9, Damage +7/+11
Soak: +12
Fatigue Levels: OK, 0, 0, -2, -4, Unconscious
Wound Penalties (-1 on total penalties): -1 (1-7), -3 (8-14), -5 (15-21), Inc (22-28), Dead (29+)
-
Runt MuAn
Shrinks Donner down to a more manageable size of 0. Doing this drops his wound range down and lowers his Strength by 4 but raises his Quickness by 2.
Base 4 R: Personal D: Sun +2 T: Individual
Power level = 10 Mastery points = 3 (25-10/5)
Mastery Points spent: 2 to reduce cost, 1 to increase speed +1
Power cost = 0
Penetration N/A (8)
Init: +1 -
My Master needs me! ReAn
Donner can instantly appear near his owner when called. The power does not grant Donner any knowledge of where Alexei is or if he needs him or not. Alexei will have to enchant powers into their bond to communicate over distance, otherwise Donner travels on his whim. Due to his natural Size, Donner must shrink to Size 0 before he can use this power.
Base 35 R: Personal D: Momentary T: Individual
Power level = 35 Mastery Points = 3 (50-35/5)
Mastery Points spent: 2 to reduce cost, 1 for Finesse 1 (Landing near Alexei)
Power Cost = 1
Penetration N/A (3)
Init: -2 -
The Inexorable Hunter (RoP: Magic page 31)
edit added Swimming and Finesse
You are short 95xp (Summer characters start out with 360xp, Donnar only has 265xp worth of abilities)
"My Master needs me" seems a bit odd. How did the dog naturally develop a power to specifically teleport to specifically Alexi? Looking closer, it seems to not be so restricted, he can teleport anywhere. That makes more sense.
Also, Donnar is Intelligent and a Familiar. Terms like "Owner" and "Master" seem a little to BDSM in such a circumstance. The Familiar bond goes both ways,. Just as he is bonded to you, you are bonded to him. You cannot "own" him any more than he owns you. You can never sell him or trade him away. He is no longer a pet. He is a Familiar.
He has an Inferiority which shorts him 50 XP, I'll have to count the rest up. The owner and master thing is something that I figure Alexei sees it as but will learn (probably the hard way) that it is not the case. The first time he tries to get Donner to not sleep on his bed will likely disillusion him of many things.
Yep, short 45 xp, I'll adjust, I use to have Survival but Domesticated takes care of that.
You know, if you had somebody already bigger than Donner in that bed, that wouldn't be a problem. 
OK, so if this looks good, I'm going to go ahead and say that Alexei binds him as a Familiar in Winter of 1228. I need to make a few adjustments then to Alexei's character via the change in xp and then add the cords to both of them.
Using Rego Corpus as the Lab Total, and that's 45, so 9 pawns of Rego or Corpus vis, i'll have to check that as well when i get to my home pc, i can't get to the records from here.
Any suggestions on the cords? I like the cords for soaking and botch resistance, i tend to favor defense.
Great, I'm not mad 
Found it yesterday. It's a little different, probably because your wishes have evolved (like, there was something about it belonging to giants or something, and I can't see me giving SS to a giant dog) but it's fun. I can search for the powers descriptions if you don’t find them.
Donar, Alexei's Familiar Dog
Covenant: Mons Electi
Magic Might: 10 (Animal)
Characteristics: Int 0, Per +2, Pre -3, Com +2, Str +6, Sta +2, Dex +1, Qik 0
Size: +2
Season: Summer
Age: 15 (15), Height: 274 cm, Weight: 333 kg
Decrepitude: 0
Warping Score: 0 (0)
Confidence: 1 (3)
Virtues and Flaws: Long-Winded (Fatigue Rolls: +3), Sharp Ears (Hearing: +3), Strong-Willed (Will Rolls: +3), Tough (Soak: +3), Puissant Second Sight, Puissant Brawl*, Second Sight, Magic Animal, Might 10 (Animal) [Form: Animal], Clumsy (Dexterity: -3 on related rolls, extra botch die), Infamous, Mute*, No Hands (Spellcasting: -5), Plagued by Supernatural Entity (Giant "owner"), Reckless, Weakness (Music)
Qualities and Inferiorities: Vocal, Pursuit Predator (Extra Fatigue: +1), Pack Leader, Pack Animal (Bonus: Fights as Trained Group with Pack Leader, Untrained Group without Pack Leader), Keen Sense of Smell Quality (Bonus: +3 using smell, +2 to Hunt rolls), Domesticated, Personal Power × 3: 75/75, Minor Virtue (Puissant Ability), Improved Soak × 3 (Soak: +6), Improved Initiative × 1 (Bite: +3), Improved Damage × 1 (Bite: +3), Reduced Abilities × 3 (-150 exp), Minor Flaw (Infamous), Minor Flaw (Clumsy), Greater Power × 1: 50/50, Gigantic × 1
Personality Traits: Reckless +3, Loyal +3, Brave +3
Reputations: Evil Monster that hunts Humans 4
Combat:
Bite: Init: +3, Attack +12, Defense +9, Damage +10
Soak: +11
Fatigue levels: OK, 0/0, -1, -3, -5, Unconscious
Wound Penalties: -1 (1-7), -3 (8-14), -5 (15-21), Incapacitated (22-28), Dead (29+)
Abilities: Athletics 4 (running), Awareness 3 (keeping watch), Brawl 5+2 (Bite), Hunt 4 (prey), Leadership 5 (Dogs), Music 3, Second Sight 3+2 (ghosts), Stealth 3
Equipment:
Encumbrance: 0 (0)
Powers:
Panic of the Trembling Heart (Greater: 50), Points: 1, Initiative: Qik-3, Form: Mentem, Range: Voice, +2, Duration: Diam, +1, Target: Group, +2, Level: CrMe35, Design: 35 levels, Base 4, +2 Voice, +1 Diam, +2 Group, +1 Animal, +1 Size, -3 Might cost
Beast of Miniscule Proportions (Personal: 15), Points: 0, Initiative: Qik-1, Form: Animal, Range: Per, +0, Duration: Sun, +2, Target: Ind, +0, Level: MuAn10, Design: 10 levels, Base 4, +2 Sun, -1 Might cost
Tracks of the Faerie Odor (Personal: 25), Points: 0, Initiative: Qik-1, Form: Terram, Range: Per, +0, Duration: Conc, +1, Target: Smell, +2, Level: InTe15, Design: 15 levels, Base 4, +1 Conc, +2 Smell, -2 Might cost: Like "Tracks of the Faerie Glow", with smell
Resistance to Claws (Personal: 15), Points: 0, Initiative: Qik-1, Form: Animal, Range: Per, +0, Duration: Sun, +2, Target: Ind, +0, Level: MuAn10, Design: 10 levels, Base 4, +2 Sun, -1 Might cost
Scent of the Predator (Personal: 20), Points: 2, Initiative: Qik-2, Form: Mentem, Range: Per, +0, Duration: Sun, +2, Target: Scent, +2, Level: CrMe20, Design: 20 levels, Base 4, +2 Sun, +2 Scent
I believe that’s what Scent of the Predator does, but I may very well be wrong :-/ It’s been a long time.
Panic of the Trembling Heart.
That’s how I see it, too.
Note also that, like Marko, I believe that, if you purchase extra size with Gigantic, it does not change your base might. I could be wrong, though.
Remarkably similar, I used less qualities and inferiorities but the end result was pretty close. Since I've already updated the sheet I'll keep him as is, and thanks again, the original idea you had made it possible.
Pleasure!
I like to design magical beasties