In the spring we are going to be ripping our my little pond and putting a huge one (can't wait....too long...must dig now!). Instead of just one large one, we are going to be putting it in sections, so I can kind of keep my fish in groups. One section for comets and wakins another for the fancies and so on. I've heard somewhere, though I can't remember now, what the maximum depth that the fancy goldfish should be kept in to avoid SBD. All of my fancies will never stay outside for the winter so I could make it fairly shallow to accommodate them...though not too shallow that it heats up too fast though.

Anyone know?

I like the idea about arranging sections for different breeds. That way you get no mix-and-match babies if they decide to spawn....
As for depth, its also to consider if the area where your pond is gonna be, is accessable for predators like cats, racoons and birds. If so, I would say, if you could have a spot where the depth is between 3-4 feet, with something to hide like a plant or so, the fish could protect themselves from those nasty fish-eating predators...

That is a great question and I have no idea what the answer is. I will tell you that I saw the hugest ryukin of my life living in a pond that was 4' deep. My ryukin is almost 8" now and it looked like a fry compared too this fish. Eventually I will add my ryukin and fantail into my pond that averages 4' deep and see how things go.
One good tip would be too make the pond walls nearly vertical. If you want too add plants you can always use plant stands or make your own stands. This way you will not have too worry about most predators.

I really wish I could remeber where I read that about the depth. It may not have even been from a reputable place. I've had a preform for a few years now that has only been 18 inches deep with no predator problems...though I do see stray cats around and we are sort of close to a river where you occasionally see herons. We have dogs, a fence, and live in a pretty tighly built area so I really really hope no one finds my pond!

The walls will be completely vertical as the whole look is going to be very contemporary. The pond system is going to be a series of squares, rectangles, and other geometric shapes and they will be semi raised. Once I sketch it up I'll post a pic of it.

I know that I read somewhere that different depths promote different body shapes in the fancies as well. That was on some Chinese breeder page I think. Hmmm, will have to research some more.