I know you said "as a medium PC," but there's a way to be gargantuan constantly that fits so well with your desired purpose that I simply have to mention it.
Aasimars and tieflings don't have to be descended humans, they can be related to any humanoid creature. Their statistics are unchanged, but they gain the size of their non-human ancestors. So halfling-descended aasimars are small, ogre-descended tieflings are large. You know what else are humanoids? Rune giants.
Non-Human Aasimar wrote:
Not all aasimar are descended from humans. Aasimars can be born of any intelligent race, though human aasimars are the most common. Aasimars of other races usually exemplify the ideals of beauty and skill as seen by their base race. For example, halfling aasimars are small, beautifully proportioned, and display exceptional grace. Half-orc aasimars are slightly larger and stronger than ordinary orcs, with tough skin and metallic claws and tusks—they are likely to be neutral rather than evil, but still display aggression and incredible combat prowess. Less common humanoids, such as lizardfolk, catfolk, tengus, and others, can also produce aasimars, though given these races’ exotic appearance, members of the more common races may have trouble telling such aasimars apart from their kin.
It should be noted that while any creature that breeds with a celestial may give birth to half-celestial offspring, only humanoids can give birth to aasimars. Thus, while it’s possible to encounter a half-celestial dragon, unicorn, or griffon, any children of such creatures would be either half-celestials or normal members of their race. (And just as often, these less conventional half-celestials are sterile.) When discussing half-celestials and aasimars, it’s important to distinguish them from both true celestials (angels, azatas, agathions, etc.) and simple celestial creatures (creatures with the celestial template, which are themselves denizens of the good-aligned Outer Planes but similar in many ways to their Material Plane counterparts). Most aasimars also have a difficult time getting people to grasp distinctions between celestial types, with common folk erroneously grouping all such beings together as “angels.”
Non-human aasimars have the same statistics as human aasimars with the exception of size. Thus a halfling aasimar is Small but otherwise possesses the same statistics and abilities as a human aasimar—the difference is purely cosmetic. Non-human aasimars do not possess any of the racial abilities of their base race. However, they are usually raised in the same cultural context as other members of their base race, and thus generally adopt the same fighting style as their peers, use the same types of weapons and armor, and study the same skills.
