The Mellennium people, as far as props are concerned, are not really in a different perspective as the standard P4 people. In the real world, people come in all sizes. There's a woman in my office who is barely 5' tall, and on another floor is a man who is nearly 7'. They can still sit on the same chairs and use the same furniture. I don't think you'll have any problem with them using the same props. When I scale an object for a character, I just scale it a couple of numbers on the dial and eyeball it. I would just try to sit the figure on the chair and determine if it's too large or too small, then keep fine tuning it until it's right. It doesn't have to be too precise, just so it looks normal. Melanie