This is a task made much more difficult by the fact that Ars Magica treats characters of different realms entirely differently. Not only are their powers all unique to their realm, even the manner in which they are constructed is unique. Magical creatures get Magical Qualities, for example, which Faeries do not. But Faeries get a number of intrinsic advantages due to their Faerie nature (like the fact that their armor and gear are made of their own bodies, so do not need to penetrate and bring no load, or the fact that they, you know, CANNOT DIE).
It sounds like your PC concept is "a Faerie Giant who was transformed into a Magic Giant." Start with the character's Might. It's probably going to stay the same, probably around 10 for a companion level PC. You can buy him the Virtues to be a Giant, and since he doesn't have any special powers beyond being big and strong, his Virtues and Flaws will probably be mostly the same.
As a Faerie, he could have had a Pretense like "Giant 6," which let him do everything a Giant could normally be presumed to do, but as a Magical Giant he will need to buy Abilities like everyone else. These will depend on Season, which will probably be Summer.
As a Faerie, the Giant could wear and use any equipment, since these items were actually all made of his glamour. But as a Magical Giant, armor will suddenly have weight again. Even an ordinary, non-Giant, warrior can end up with a very high Soak from armor. When you combine this with the Giant's huge Wound Levels, he should be far and away the most durable fighter at the table, though because his Defense is penalized by his low Qik, he will be vulnerable to high-skill champions.
I guess this is all a long way of saying, "pretend the character kept his Might from his original realm, and then rewrite him from the ground up in his new realm, sticking more to the spirit of the original creature than any strict conversion." The fact that creatures of each realm are made using entirely different rules makes any conversion a once-in-a-campaign story element.