Regarding Hunter sizes, I recall reading in Contact Harvest that Hunter worms individually have no intelligence, operating purely off some basic instinct. However, when the worms are proximate to each other, they gain a greater collective intelligence, especially if they are in physical contact. The more worms in the same place, the greater the intelligence. How many worms can be together in a single frame depends on the inverse square rule for mass and strength, the local gravity, and how strong the frame is. Thus, a low-gravity world can have very large Hunters, while a higher gravity world will have smaller ones. Covenant Scarabs are actually huge Hunter frames, technically making them a giant Hunter form.
However, there is also a minimum mass of worms that is necessary to achieve a degree of intelligence we would consider to be sentient, which limits how small Hunters can be and still be effective. This problem can be mitigated by having several smaller Hunter forms maintain proximity to one another, allowing some sharing of intelligence between the different forms, making them collectively smarter than any one frame would be as an individual. This is why it is common to the point of ubiquity that the "humanoid" Hunters typically encountered in the game are almost always deployed in pairs. This is also why one goes berserk when the other is killed, as not only did it just lose a piece of itself in a very mentally and emotionally stressful manner, but it also reverts to a lower, more beastial state of intelligence and can think of no tactic more complex than "Charge!"
As for detailing, I say go ahead and base your Hunter costume on the Halo 3 forms, but add what Reach detailing you can see to it. There is no need to wait on Reach to show you better sceeenshots. From what we can tell so far, the scale and detail on the Hunters has changed, but not the form. It is essentially the same as the older ones, just bigger and with more elaborate bits on it. You can start with the older designs, and just add to them as you get new information.