in the wild they do get much larger, ive seen a 8in percula clownfish in Australia before. but i dont think they will be any different in a tank, but they might be a little more energetic and colorful.
wild or not in a tank they will only do so well/grow so large. You can tlotally see an 8 inch clown in the ocean, but wild or tank raised you wont see one in a home aquarium...so why take one outta the wild if you dont need to.