I'm slowly populating my Finnish Saga (that currently lies dormant) with poweful Magi from various houses, living in different Covenants spread over the entire Scandinavia, but mostly close to the south-western part of modern Finland though. The Magi that I'm doing stats for are the ones I intend to use as champions of the specific Covenants, and thus should be dangerous Magi with Hoplite potential. And what better place to find inspiration for those Magi than in here 
Based on the great 'PerdoBear' concept of Argmark, I have created Bjartskular of Bjornear. Going through the Bjornaer section of HoH:Mystery Cults I found some further inspiration. He is a prominent member of Clan Wilkis, in the role of a Hunter, striking hard on those who invade the Wilderness with the intention of taming it. He has done the three rites of that particular Clan gaining the flaw Infamous and Vowed to strike against the Christian Church; while gaining the virtues of Berserk, Cautious Sorcerer and Life Boost. I choose that particular Clan because it fit how I pictured the character well, and also because of his role in the story I'm creating where the Magi of different Covenants rise up against the invading Christians. Boy was I happy to see that the virtues/flaws fit the concept ever so nicely too - Collateral Munchkinism? 
During my browsing through the sourcebook I was unfortunate enough to stumble upon a section that clarified how the Size of the man does not affect the Size of the beast. The Giant Blooded bear is likely to be bigger than any other bear, reaching the very peak of the 700 pounds wieght limit, but he would just be Size +2 nevertheless 
I have chosen to ignore that for my Saga though, since the book later gives the SG the option to up/down the Size limits as he sees fit. If you are going for pure Size in order to increase the wound categories (and damage modifier) you could consider the Aurochs, which is the biggest mundane animal available with its Size +3. I haven't found any stats for it though meaning that you would have to design it youself using the rules listed in the very same section of the book. This, however, means that you could inflict some of your Munchkinism on the mundane part of the beast as well 
Also, it seems that a mundane bear already has the virtue Tough, and to rule that they would stack is stretching it I'm afraid 
(I'm assuming the Toughness virtue was not for the human form anyway). Instead I would heavily recommend Enduring Constitution for a design such as this (as well as for my own Trogdor design above). First of all, it is a boon to nearly anyone using fatigue for spellcasting be that Fatiguing Spontaneous Casting, Life Boost, Life-Linked etc. since it's an almost sure +1/+2 to everything in times of stress or combat. At the same time it's potential for bonuses (or more precisely, it's negation/reduction of penalties) in relation to wounds is incredible! Since you are not very likely to kill a creature with very high Soak AND broad wound intervals with a single attack or spell, you would often be forced to make it accumulate wounds. But since anything below, say damage 20, results in a Light wound at no penalty - this is not likely to work either. With Enduring Constitution, after a minute or two of fighting the PerdoBear might have suffered a handful of Light wounds as well as spend 4 fatigue levels, without even suffering any penalties at all! Throw in a couple of Medium wounds and a Heavy wound and you will have saved yourself from a dozen points of penalties. Thats alot for a Minor Virtue 8)
