I guess a Medium porcupine would seem pretty big. Want to add any more special abilities, or just make it bigger and stronger? Any good prehistoric inspiration?
How about making a 3 HD Medium-sized Dire Porcupine which advances to Large at 6 HD? That would cover both our bases.
That's fine with me. So is the dire porcupine supposed to be a giant porcupine conversion, too?
Yep. Most "giant" creatures in 3e are magical beast, often intelligent, versions, like the giant eagle and giant owl. The rest became dire animals.