IMO, there's still a single 'target' for Wizard's Icy Grip, it's the 'heat' property of the target. From a Mythic European or modern Physics perspective, the average living human/animal body is outputting far less heat that a good-sized campfire, which is the standard Ignem unit of measurement - by multiple orders of magnitude.
This spell, and several others, seem to get a slight pass compared to other arts in terms of casting requisites, possibly because Ignem is a more advanced or developed Art historically. For example, unlike it's PeAq equivalents, no casting requisites are mentioned for this spell.
Those are noted specifically as having casting requisites for the targets; which makes them slightly tricky. I'm not sure if technically adding in the Animal or Corpus requisite also adds the Animal or Corpus size limitations - though I would guess so.
After thinking about this more, and reading the replies, I think I would simplify further and simply utilise a General guideline for damage of Base 5 for +5 direct damage, ignoring armour soak. Each magnitude adds +5 damage. I would generally disregard size modifiers and form prerequisites - yes your base PeAq10 spell can do +10 damage against creatures that require water to survive, just the same as a PeIg cold spell or a CrAq throwing acid at enemies. Creo Ignem gets a bonus on this guideline of an additional +5 damage, as it is the most damaging combo.
Its very easy to remember (Base = damage bonus) and lets everyone have something to offer in combat; but at the same time, reminds us combat isn't the central theme of Ars Magica.